I could join the ranks of bitter young men here who think women are just out to fvck with your head; truth is, many young women are, but so are the majority of young men. At your age, do you think you would have stayed with her had someone more beautiful, more interesting, and smarter come along? I'm not calling you a cheater or demeaning you in any way, but in a short term relationship oftentimes it's easy to get one-upped. It's the way it is.
That being said, you need to move on. She'll compliment you like she did for two reasons, at least in my experience: first, to legitimately soften the blow. Second, because she honestly believed it at the time (at least some girls are like this. Hell, I wrote an email or two like this in high school relationships). Or, if she's slightly more malicious, she'll write this even towards the end to string you along.
In my short time on this earth, I think I've come to believe that most young women aren't different than men. We want to be loved, we want to feel special; we want that butterfly feeling. We want to be with the best person we can. That's part of being young and changing and growing. Neither young men nor young women really know what they want, in my opinion, as I've been tempted to do the same sorts of things when I've been in relationships. It's very easy to say you're committed, it's another thing entirely to live that out over a year, two years, or heaven forbid a lifetime. Women just seem worse because this is a predominately male forum.
My mother got screwed over royal by my father during their divorce and beforehand; he's a man who's now in his fourth (and, please God, his last) marriage. He's my father, and I love him, but I realize that he has his own issues and he's made his mistakes, and that he's hurt his share of people in his life. Some people are just that way, and I've seen enough sh!t from both genders to say that's not limited to the fairer sex.
Now I'm just rambling, but I hope my import is clear. Get over this, go running, lift weights, or hell, just find some lucky girl to fvck for a few weeks. Life, and love, move on quickly.
Cheers!
Nate
Nice guys DO finish last these days 
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