sounds like alot of us feel the same way and have had this happen to us alot. Mine is about 3weeks now, we dated for 2 yrs ... monday she said lets get married ..i said i need more time she was 2 young 20 i am 25 i know there is lots to learn in those few years. But anyways back to you.
Get angry, dont call her ... write her and try not to think about her. I went to buy some smokes today and saw her car at a guys house she cheated on me with ... not the guy she is engaged to 2 weeks after we broke up. Now mine as a nutt case.. a total freak i should have left her years ago... but wanted to be the nice guy ...dont be that guy. let it go, move on. It might take you a few weeks months maybe longer ... but move on.
women are very strange, not much you can do after they want to break up. they have made up their minds and are not going back. now 1 yr from now you might run into her and all that might change ....
get a hobby, go to bars (dont see her everywhere you go!)
do stuff
start hanging out with your friends again ...
it will be ok... trust me
good luck