Read up on "Rejection Sensitive Dysphoria", it's a common bedfellow of ADHD and leads to "catastrophising". Alternatively, it could just be that you have a pessimistic view of the world that is grounded in reality. Only you know which is which. I'm a bit older and I have friends with serious illnesses and for them it's very real. In the meantime, I'm still having adventures and planning my life years ahead. Thinking positively won't stop the bad stuff from happening but it will mean that I use the time I do have left as well as possible.