Your doctors say that you are too young to have that, and you know
that you are not? Finished medical school at 17?
For the anxiety, you should really consult therapist. You are young, you should enjoy life, and not live it in fear. So, first, threat the main problem which is anxiety.
As for your leg problems, you didn't mention what doctors said it is? Ok, they dismissed terminal illness, but did they come to some conclusion what you could possibly have? Maybe some boring no terminal disease?
For now, try to relax and occupy yourself with things that could distract you from thinking about the illness.
Finally, remember that even though you may feel that you have some terminal disease it doesn't make it true at all. Those are just emotions caused by too much anxiety.