I really sympathise with your situation - been there.
What you are finding is that the two strands of the human condition are working percectly in tandem -
your rational mind is making lists of reaosn why this guy is wrong for you, and you are listening to that side of your mind, and knowing it speaks the truth but -
your emotional mind is ignoring all that and giving you a terrible time as it obsesses over someone you can't have, don't need, and in all probability, would hurt you - but still it carries on and -
guess which side of your mind gets to occupy your weaking seconds???
So - what to do.
If you are sure you don't want to try and form another relationship - which is sensible, you are not in the right frame of mind for new starts right now, you need something to get your head away from this man.
What about getting an evening job - bar work is always available - if you are bright enough for uni, that will be no problem. It gets you into a new circle of people, occupies your time, and even makes you a bit of money.
Try socialising on a superficial level with anyone you know - cinema or a night out somewhere, anything to break the circle of thinking and fretting - doesn;t have to be anything serious, in fact, the lighter the better for you.
All this will at least give your mind some time off from this situation, and once you get to uni, you will have far too much to do and think of to have time for him.
I know it's a wretched situation, but it will pass, promise, and you will get past this.
Keep thinkin that, and telling yourself that, and try to keep yourself busy physically and mentally.
Best of luck with this, and post here if you need to, any time.
A x