I've been there before. A few months ago, actually.
I chose to break it off because it wasn't the right time for me. Was it hard to do? You bet. Do I regret it? Not a bit.
What I've found is that WHATEVER you chose in such a situation isn't nearly as significant as we make it out to be.
Anytime I've been truly terrified that a girl I'm dating is irreplaceable, I've managed to find a replacement. This isn't dumb luck; it's the reality that there are no soulmates.
Of course, at the the time of any breakup, the idea of losing someone forever sounds so dramatic and life-altering that we naturally put off such a decision as long as we possibly can. One hundred years of Western cinema (and nearly one thousand years of literature!) has certainly beat us over the head with the concept of lost love, and making a decision to let someone go is incredibly painful.
Once it's done, though, you'll find that our hardwired adaptability, if you will, kicks into gear, and any doubts you once had disappear. Sure, you'll think about her now and again, and it'll pain you a bit, but you'll get over it.
The funny thing is, according to my married friends, you will go through the same process if you decide to MARRY her. Meaning, you will adapt to the new role that is required of you, and accept your lot, so to speak. You will NEVER be ready for marriage, because lifelong pair bonding isn't really a natural thing for us. There will ALWAYS be doubts, and when you are married, you WILL wish you were single from time to time, and you WILL get sick of sleeping with the same person. That's just life. But, no matter which path you choose, your happiness still comes from within. The path you choose is only "life-changing" because you have NO WAY of knowing the alternative path.
It seems very anti-DJ to say that it doesn't matter if you marry her or not, but it's important to know that you will adapt either way. Obviously, if there were major red flags involved, I'd say let her go and don't look back. But, this is obviously a tough call for you, and I respect whatever you choose.
My own decision was to take the most conservative route: A man can get married at nearly any age, so why make such a commitment in the first third of my life? I figure that life is more interesting for single folk, and I'd rather wait until I know I want kids before making any kind of commitment. Despite this, I know that if I chose to get married, I'd get used to it. She was a good girl, so I can't say that I "dodged a bullet."
Whatever you chose, you're gonna be ok in the long run, so relax your mind a bit. Don't do something just because SHE wants it, though - this decision should be about what YOU want to do. Don't feel guilty if you're not feeling marriage just yet; she'll be ok in the long run, too.