Hi there,
I'm sorry to hear what you've been through, but equally amazed at your strength and tenacity in going on to complete your GCSES - and come out with results that anyone would be proud of!
As regards your feelings of worthlessness, I can relate in the sense that I, too, for some unknown reason, have always put pressure on myself to do what I consider to be my 'very best'. I honestly don't know why - I never been nagged or pressured by my parents, but I completely understand what you mean when you say you feel dissatisfied with your performance, even if you have done what others would consider 'brilliant'.
I would say that your position on the transition from GCSE to A Level is shared by many. All my friends commented on the increased difficulty of A Level study, so I wouldn't worry about it. If you're finding the change particularly difficult, to the extent that it's interfering with your studies and/or general health and wellbeing, I'd definitely recommend that you speak to someone. Tell whoever you feel comfortable talking to - your parents, relatives, friends, college tutors, welfare support staff, etc. They'll all be able to help and support you - particularly at college, where they can implement additional emotional support alongside your studies, if needed.
In relation to struggling for time in your exam, this is another common concern for students - you're definitely not alone on this one. I would say this issue is by no means a reflection of your academic abilities, but rather more to do with time management and technique during exams. I had the same issue at A Level, but noticed a vast improvement in these particular areas after seeking advice and tips from my tutors, as well as attending revision sessions and doing practice papers to help sharpen my skills. It may be that you're entitled to extra time in exams, though that's something you'd have to query.
Coming back to your general feelings, I wonder if it might benefit you to see a counsellor to discuss exactly how you feel and get a professional perspective on things? I'm currently having counselling and find great comfort in the reassurance I receive and simply having an outlet for my queries/worries/doubts.
Most importantly, you are most definitely NOT a failure. You are clearly a bright, intelligent person who has overcome and achieved so much.
Wishing you every happiness x