How can I work with doubt and existential concerns? After a 'bad trip' a big hindrance of mine is doubt and the idea that all is just a dream that we have no control over. This brings up terrifying ideas, concepts and images for me. That this may all just be nonsense, even buddhism. I'm aware these are concepts and mental activities, but the panic that they bring on can really deter me and interfere with my practice.

I fear that the Dhamma may just be a temporary fabrication of this reality that I'm in etc. The fact that I suffer and endure loneliness allows my mind to go astray. I'm not a member of any sangha and I dont get any support to continue to practice. I've suffered with misery most my life and this fact brings up many fatalistic fears. I'm wondering how I can try to start overcoming this and progress in the practice. 

Thanks
Meditation is the key to beginning to understand your mind. The thoughts, images, indeed all mental activities are not "you". Meditation (in just about every system, religious and not) is about discovering the truth about your self. (The word "self" has many completely different meanings depending on the context of the word so don't get too hung up on the word.)

I suffer from depression. It is my experience that depressive episodes come and go. And I remain. Fear comes and goes. And I remind. So, meditation on who I am (and am not) is more helpful than meditation (or attention) on that which comes and goes. As you practice meditation, the mental noise will decrease. (Thankfully.) At worst, you give your mind some rest. Try it. It works. One day at a time. Good luck!