How do I tell my partner I'm not ready for a long term relationship?

I’ve been together with a woman I respect and care for a great deal for almost 2 years now. We’ve lived together for only 6 months. I’ll be going back to university in 6 months time and we were thinking of getting an apartment together in another city. It’s a big decision because I feel I am still immature, somewhat, and don’t want to settle down with her just yet. I want to continue meeting other people, I suppose. It might stem from the fact that deep down maybe I have doubts about the 2 of us… How do I tell her I don’t want this long-term relationship to go on without destroying her life as it is now?