How can I ever enjoy life when I constantly doubt that I am really alive?

You say its not a phase because it is triggered by things that really happen, yet the way you react or perceive what happens depends on your age. Im 19 now, and my way of seeing my life has varied a lot through the years, i used to be suicidal, then it got better yet i also doubted i was alive, not the same way you do, i didn't doubt my individual existence but that of others haha and i still believe it, but now i see that whichever way i perceive my life it still happening, im still in the middle of this, real or not, and i can make it be wonderful if i want to, even if you don't exist (which you do) you are still living your life and you still can make the best of it, think of all the experiences you have left to live.