When I was in my teens, about 25 years ago, I used to experience severe panic attacks that related to my own concerns about reality, the afterlife, and other questions of what could be existentialistic in nature. I agonized over questions, just like you. I come from a very religious, Catholic family who went to Mass every Sunday. I asked myself if reality was indeed a part of somebody else imagination, or if it was truly "real." A psychiatrist told me something that is attributed to Rene Descartes: "I think, therefore I am."
Fortunately, my faith also helped me to simply 'let go and let God,' something that I've learned in Al-Anon and an Adult Children of Alcoholics' support group in the Northeast. My medicine helped and so has the therapy that I've received.
Things get better in time.