I know where your coming from man... wanting to spend on your car but at the same time trying to balance the little cash you have left. But I'm actually on the same boat as you. I'm 19 currently working atm, trying not to spend all my money on my S and trying to invest in some property or save for the future, but I too am thinking of leaving work and studying for the exact same reason =p.

To be honest though, you've spent so much on your car that it's not really worth selling (because of how much you'll lose). But I understand it's hard to make a choice whether to let it go or not. How bout you just fix what's left to fix and keep it, work for a couple more months to save a bit more cash then decide whether you really do want to continue studying.