I know that it's easy to get freaked out about possible medical conditions. I've been there too. Ironically, it's often that fear that pushes us towards drinking and drugs, which as you said, makes things worse.
I'm glad you have an appointment with a therapist. One thing that helps me is to remember that none of us know what tomorrow will bring. We really only have the moment we are in now. Focusing on that can really help. Have you read 'A New Earth' by Eckhart Tolle? It's gives a good plan for how to live in the moment and let go of everything else.