As daft as it seems, this was my PC wallpaper all the way through the time up to my finals, as a reminder whenever I just knocked the PC on, of what my priorities really were.
http://browse.deviantart.com/customization/wallpaper/minimalistic/?q=work&order=9&offset=24#/d214sft
There are a bunch of questions to ask in this situation though
1) Is academics the best thing for you? If not, stop revising, get an apprenticeship
2) How have you revised in the past? Whether this did or did not work for you, learn from the techniques you've tried
3) You learn most by actually doing. Don't just read books, write things out, try questions, whatever is relevant. The amount of paper I used to get through when I revised was obscene.
4) Push yourself, and reward yourself. There are times in your life when you may have to push yourself to breaking point. It's up to you whether in your life, your set of values, this is one of those times. Once you've understood what this hurdle does or does not mean to you, reward yourself accordingly when you stick to your guns.
5) On all of the above, get an outside opinion. Speak to your tutors, your college career advisor, Old Bob who has known you since you were a wee lad, whoever, but get an independent opinion of you, and your situation.
Just remember, if you commit yourself to AS/A level exams, you need to do well, because it's a tough world out there. HOWEVER, these are not the only ways to succeed in life. I work for an international firm with a turnover in the billions, and it's not uncommon to see top executives that have come up from being an apprentice at 16.
Hope that's of some help to you matey.