ooh tough situation.
One thing though to keep in mind with these long-term/thinking-of-marriage relationships: there is nobody who is 'perfect' for you. There is no 'soul mate'. There are literally thousands, maybe even millions, of women in the world whom you could marry and be happy with. Even the best marriages have their rough times. Finding a perfect person is impossible, but many people make it work and are, for the most part, very happy with non-perfect mates.
Does this mean that you marry the first girl you come across? No. But, when you find a girl that you get along with pretty well, and you go out for a half-year or so, and you really can't think of anything that you really dislike about her or that you see coming up as a problem in the future, well, you've gotta make a choice sometime man. You can't continue your search indefinitely. You've got to compromise. I personally like breasts very much, but if I found someone whom I couldn't fault in any other way, but her breasts were terrible, I'd still marry her. My chances of finding somebody like her in every way except with better breasts, are virtually zero.
So you've always got to weigh what you have now versus what else you could find out there, and what are your chances and how long it would take you to find it. Is 90% in the hand worth 92% in the bush? (no double entendre intended) At your age, maybe it's best to break up with her since you have time to find someone else. But also you have to consider that you're 20 and this is your first girlfriend. How long do you think it will be before you find another girlfriend?
So I suggest some serious thinking about what it is that you want in a wife, and whether this girl has any problems that disqualify her. If you come up with anything, then break up with her. If you can't come up with anything, you should stay with her, until you find something that you decide you can't spend the rest of your life with.
If you don't want a wife, then that's a serious thing that you should tell her. If you want a wife but just aren't ready to decide on that kind of thing right now, tell her this as well. Make this fear of commitment (maybe I'm overstating it a bit) your problem that you can share with her and that you can work on together, just as you work on her emotional problems together. Let her know that it makes you uncomfortable that she's always asking about a ring and stuff, and that you need some time before you can start to talk about that stuff. Tell her to be more accomodating to your needs, because even though you're relatively normal with respect to her, you still have your shortcomings.
Best of luck.
wbwither