How can I work with doubt and existential concerns?
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!