Constant mindfulness hard to achieve I find very hard to be always mindful of all activities (four frames of reference).

At the beginning of practice, my persistence are aroused and gradually it becomes indifference/lax. Basically I am starting feel stress trying to be mindful..?

Since it is necessary to have other factors like joy, calm (7 factors of enlightenment) to achieve wisdom, I assume I am doing something wrong.

What I have learnt from sutta is 'sati' dhamma is cause for arising of other good dhamma.
So what are the conditions to have this 'sati' ? .
When you start of you need some level of concentration to have awareness. The reason for this is we are constantly getting input from all 6 sense doors and we react to them. So the starting point would be to focus on narrower area in terms of input and be persistent.

When it comes to awareness what you really have to be aware is the sensations caused when any input comes through sense doors, and to be equanimous towards it. This is mutually reinforcing. Once you have concentration and equanimity (you need strong morality as a base) your awareness improves. Also one of the main constituents or our frame is sensations so it is always there. All Karma gives result as means to create some sensation (e.g. you break your leg then this is pain. When you initially plant the seed of Karma also you will feel similar pain but at a much lower intensity than when you reap the results. Many of us are not sensitive enough to see this.)

You will naturally get stressed if you practice the wrong type of awareness. E.g. and acrobat will have a strong sense of awareness and some level of stress to maintain this awareness. More longer you do your gig more stressful you become. The right type of awareness does not give stress. This is being alway sensing the sensation in your body and to be equanimous towards it.

E.g. when you are walking, you use your faculties look at the process of you walking. Through your faculties you can sense pressure on your feet and certain change happening with regard to moving giving certain sensation. Also the impulse in you mind to more of what sensations this triggers.

The process that creates stress is when you react to sensations with clinging. If you are not equanimous, you will cling causing stress. Also if you miss sensations you will miss seeing this process where you react and cling to sensations.

Once you start this way a point will come you can sense sensation in your whole body and see how this reacts to different inputs from your sense doors. Though this would will see that your body is also changing. You see the sensation of liquid nature, solid nature, gaseous nature, temperature in your body. All made of tiny Kalapas arising and passing away.