Alot of peopel you may think are successful have problems themselves, or have been through similar problems that make themselves stronger.
As someone said, you have to have a positive mental attitude. Try to stop spending time worrying about your exams, and focus on doing the best you can on them

. N worst the worst that can happen if ya dont do well? like nowadays, you can retake them, or try other qualifications.
Some of my friends have done amazing from getting D's and E's at A level.
Maybe you need to stop thinking about the future.
I had this 1 career in mind from when I was 13 to 19, and now I'm 20 I've changed my mind, and at first it got me down, but then I realised that I have so much to offer to whatever I do, that it no longer freaks me out.
So try and think positive, as you have something to give the world, even if you dont know it yet.
Good luck.