Do you have dyslexia by any chance? Anyway I think like many I have been in your position of wondering if I can ever be good enough. Yes, there are certain things that require a high skill level that you might not be able to achieve (as stated above). However, if it is to do with A-Levels then I think it is possible. Speaking as a dyslexic, it might take you a bit longer to achieve your own goals but it's ultimately about the effort and hours you put in. If you have picked subjects you enjoy, then it is much easier to put the work in (personal experience but might not apply to everyone). Additionally, remember that intelligence is more than just A*s. I know that last point was sappy but it is true.