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Imagine determinism as a continuous chain of actions. Each action at a previous moment in time fixes an action that happens now, which in turn fixes an action that will happen in the future.
If this is true, then your personal actions are part of this deterministic chain and are what creates the future (the next set of actions). 
Now the kicker. You said

Its not in your control anyways. Since there is a good chance that free will doesn't exist, should we really worry about anything?

This is a roundabout way of saying: The same future happens, regardless of your personal decisions and actions. You have the same destiny, no matter what you do.
However, if determinism is true. Then this outlook doesn't make sense. Because given that your decisions are part of what fixes the future, how could it be said your decisions were pointless, and that future you did/didn't want happens anyway? Saying that would be going AGAINST determinism because it would mean your personal actions aren't part of the chain of actions that fixes the future. The fact that your exact actions were foretold by a Greek oracle thousands of years ago isn't really relevant, because you don't have access to that information.
The person mugging you has no choice, the you being in pain has no choice but to suffer and worry. If you have no free will then you have no choice over whether you worry or not. >

No, you can't know that the me being robbed feel suffer and worry. It's also possible that I won't care, because I know I can't choose to feel bad. It is, as it has been mentioned, already determined, what I will feel. >


you are not responsible for procrastination, for being lazy, for not contributing to society, being unkind, and everything else. Its not in your control anyways. Since there is a good chance that free will doesn't exist, should we really worry about anything?  

Well, you're in a society where your actions have effects and create reactions, whatever you are responsible for it or not. Steal, and you'll be jailed, whatever you did it because you are evil or because you were socially predestined to crime. Eat 2kg of meat each day and you may get a heart attack, whatever you are responsible for such a choice or not. 
Some outcomes (being rich, being happy, having a beautiful & intelligent partner) are better than other (being sick, being lonely, living under a bridge) so of course you should worry about what you do, even if ultimately, you were predestined into worrying about them and therefore obtaining what you get. >

A lack of a belief in free will does not necessitate adherence to fatalism. Even non-believers of free will acknowledge that you can’t make an omelette without breaking some eggs. In the same way one can acknowledge that even if ultimately our motivations are simply handed to us via unconscious processes and aren’t “free” in any meaningful sense of the word, it is still sensible to apply our agency in ways that lead to better outcomes.
Fatalism and lack of belief in free will absolutely do not need to go hand in hand.
Edit: Typos and shit, I blame my mobile phone. >

i don't get it, if we don't have free will how do we "apply our agency"? >

Well, that's just simple, isn't it?
Say you have the craving for an omelette. You can't really account for why this is the case; you don't choose your desires. But here we are. Now, not only do you want an omelette, you also decide to go and make one. Again, a decision handed to you via unconscious processes you can't really claim any meaningful control over. So there you are, making an omelette. And when finished, there you are, enjoying an omelette. That is agency; an end is achieved via your actions. 
Surely you're not suggesting that without free will, we'd grind to a complete halt. >

So are you arguing for fatalism then?
Why would anyone study for a test if everything was predetermined?  If I am getting an A, then I am getting an A, regardless. >

essentially yes. I was surprised to learn that many scientists didn't believe in free will (many biologists in the lex friedman podcast). >

Switch the word responsible with representative.
We aren’t necessarily responsible for our actions, but we are representative of actions within the environment. Thus, systems of punishments and rewards are designed around the accumulation of events in which you emerged, because you embody and represent a complex organism that is influenced by the environment. What you should worry about is the type of environment you are representing.
Our agency extends only to the environment that surrounds us, which then feeds back into who we are and how we behave. >

Assuming there is no free will, you still will worry if that is what has been already been predetermined from the starting conditions. >

assuming free will exists, we should act like it exissts. If free will does not exist, then acting like free will doesn't exist results in not criticizing anyone's actions, thus ultimately lawlessness.  thus we should still pretend like it exists. >

If we don't have free will then I don't have a choice about whether I do anything or believe anything that will reduce my worry.
If there's no free will then I have to do everything that I do because I couldn't not do it. If I worry then it's because I was determined to - I HAVE to.
I'm not taking a position on whether or not we do have free will here, but your OP is wrong. If we don't have free will and we worry then we HAD TO worry. >

not only are you not responsible, you have no say in the matter... but at the same time you also have no say if you "feel" responsible or if others (also under no will of their own) seek to punish you.
The free will position has a pretty hard time as far as a lot of empirical science goes.  My personal position however comes from considering what it actually means for there not to be free will
If we truly exist in a deterministic universe then I think much of what we would consider the best parts of living cease to be. We would have never made a decision, only had the experience of making one. We have never learnt anything or developed as a person. Worst, we exist entirely alone and in isolation because communication between minds (assuming other minds exist) is impossible. Every word or gesture you have ever made never really came from you, and no word or gesture directed at you came from another mind directing it. All attempts to interact with the world or with other individuals are meaningless. >

What do you mean by "free will"? 
It's not a monolithic thing, there are various versions of free will. For example, libertarian free will, compatibilism. >

I don’t believe in free will, yet I don’t think that means we don’t have to actively participate in life and take responsibility. 
Yes, the choices we’re going to make are fixed, but that doesn’t mean that those choices will happen regardless of whether we choose them or not. Participating in life still matters because while there’s only one way everything can happen, we don’t know what that way is, and it’s better to behave as though we do have free choice. 
No free will doesn’t mean different outcomes aren’t still because of human behavior, we can’t check out and not worry about anything >

We can simultaneously have no free will and have some free will. Just like if you walk down the equator you are going on a straight line, but that can also be a circle or an oval depending on the angle you look at it. >

You still suffer consequences so you have to worry anyways. >

Let's say that there is a parallel universe where another version of you worries about nothing and as a result, lead a miserable existence whereas the sane version of you in this universe worry about certain things and lead a much better life.  Both universes are deterministic.  Would you agree that worrying has helped your version? >

If there is no free will the idea of what is logical or moral to do does not make sense so the phrase "If there is no free will then people should. . .," doesn't really make sense either. This is mostly semantics but I feel like you are having a hard time getting into the headspace of not believing in free will. You don't stop having feelings and goals and there is still cause and effect, things still happen to you and your actions still have consequences.
Free will has a lot to do with identity. What actions can you say are yours, are of you? And if you understand yourself to be a result of matter and the physics that govern it and you believe that large scale physical phenomena can be predicted by the events that preceded it then your actions can be predetermined. I personally believe this to be true but that information is not useful to me in a day to day sense so I operate in ways that don't account for how my "choices" arrived to me in the same way I believe the world is round but I never think about it when I'm just walking down the street. >


If this is true, then - you are not responsible for procrastination, for being lazy, for not contributing to society, being unkind, and everything else. Its not in your control anyways. Since there is a good chance that free will doesn't exist, should we really worry about anything?

Isn't worrying then also part of the already predetermined behavior? It makes no sense to suggest that we shouldn't, if we can't stop it. >

but by reading this post (the input), some people will no longer worry (the output). Of course, they didn't "choose" to read this post, but that will be the sequence of events. >

Switch the word responsible with representative.
We aren’t necessarily responsible for our actions, but we are representative of actions within the environment. Thus, systems of punishments and rewards are designed around the accumulation of events in which you emerged, because you embody and represent a complex organism that is influenced by the environment. What you should worry about is the type of environment you are representing.
Our agency extends only to the environment that surrounds us, which then feeds back into who we are and how we behave. >

I think the idea of free will only concerns what can be controlled, which is the self and not others' perception of the self. The specific items mentioned e.g. procrastination, laziness, contribution to society, and laziness, etc are values created by a society. They are not intrinsic properties of the self. So free will or not, those cannot be controlled. I believe Taoism, Stoicism, and the Serenity Prayer all talk about letting go of what cannot be controlled. >

Yes and no. It is factually correct that there's no free will but that doesn't mean you should throw your decision making into the pan and just disregard any capacity you may have to influence the course of events in your life. 
Sure, after the fact, you couldn't have done anything else as you had no free will. But the problem, is while you're deciding, you can't know what the outcome will be. You still have to weigh in your options with the information you have at the time and contemplate long-term or short-term consequences etc. Whether you "really" had a choice or not doesn't change that. 
In short, it's best to act as if you have free will even if you don't. 
But I definitely think we need to be more forgiving to others and ourselves for past "mistakes". That's the most useful lesson from the realization that we have no free will. 

If this is true, then - you are not responsible for procrastination, for being lazy, for not contributing to society, being unkind, and everything else.

None of these things are universally "bad" the way you might think. Laziness for example is underrated. Conserving energy is an important tool. >

Switch the word responsible with representative.
We aren’t necessarily responsible for our actions, but we are representative of actions within the environment. Thus, systems of punishments and rewards are designed around the accumulation of events in which you emerged, because you embody and represent a complex organism that is influenced by the environment. What you should worry about is the type of environment you are representing.
Our agency extends only to the environment that surrounds us, which then feeds back into who we are and how we behave. >


Assume that there is no free-will. This means you don't really make decisions, the decision is already made and its just presented to you, giving you an illusion of choice.

This is a non-sequitur. You make decisions. It's called "thinking" and it's real regardless of whether some poorly defined religious construct of "free will" actually exists