It sounds like the OP is suffering from "grass is greener" syndrome. This is normal.
He's talking about all the amazing parties and casual sex he's going to be enjoying at university. Well, maybe it will be like that, but maybe it won't. He thinks that if he goes there in a relationship, he'll be missing out on these things; but maybe he will even if he's not in a relationship! It's normal, when in a relationship, to think longingly about all the other people you're not able to sleep with and all the things you're not free to do, but your view of these things is distorted. The unfortunate truth is that if you weren't in a relationship, you probably wouldn't be doing all those things anyway.
This is something you learn through bitter experience. The OP simply hasn't learned it yet, and this is no doubt partly because of his youth and partly because he's found himself in a serious relationship at an early age and hasn't had the opportunity to live the single life and see what it's actually like. If you do break up with this girl, it doesn't mean university life will be an endless round of debauched orgies. It just means that you won't be with her.
Similarly, single people often fantasise about what it would be like to be in a serious relationship, and they idealise it, thinking it's all romantic dinners and amazing sex and cycling through the tulips and what-have-you. It isn't, and when they get into a relationship, they have to learn it the hard way. We all imagine that the grass is greener on the other side, but it isn't, it's just different.
Splitting up with someone because you'd prefer to be single is usually a big mistake, because being single isn't actually what you think it is, and so you're making a decision based on faulty information. So I would advise the OP not to do it. However, he's unlikely to take that advice, and perhaps in a way he shouldn't, because the only way you can really know and understand what I've just been saying is to live it. Unfortunately I can't see any way for the girlfriend not to get horribly hurt by that. Overall, the advice many others have given, to talk about it honestly with her, is probably the best start. You may find that doing this strengthens the relationship and makes you not want to leave it after all.