I agree with MickyCheeky @NeedHelp104 no time spent studying is wasted. That is something it took me a long time to realise, but you are still very young, sometimes these things don't become apparent until we are older.

You are dealing with a life changing diagnosis. Do you have support from a therapist or anyone like that? There are therapists and psychologists who specialise in helping people who have diagnoses such as yours. Sadly there are others who are dealing with similar issues, but that might help you feel less alone.

What you are feeling is valid - I know I would feel cheated too (& I never was such a promising student as yourself) I think anyone would. But coping with this change in your life can be a process and professional help could make a big difference.