Study your head off, you might as well. If you just sit there doing nothing, in the summer holidays you won't enjoy it because you would regret not studying. There is still time to change and get into a decent uni. Drinking actually makes you feel even more crap afterwards. Fact.
As for the girlfriend, break up with her ASAP. The longer you hold on, the harder it is to do it. Once you've done it, that's a load off your mind.
I also don't have any real direction in my life. I don't think most people do have a specific direction in their life. Usually you decide that later, but as for now, do what you enjoy most and when you go to uni, you'll probably have a bit more of an idea of the sort of thing you want to go into (if you're talking about careers).
Motivation is difficult, I guess a way you could do it is to scare yourself into working hard. If you don't get your arse into gear, it's your own damned fault! Why do nothing when it will make you feel pathetic later?
Speaking of which, I need to do revision.
Snap out of it! For your own sake.