Feel your pain here myself bro. Just got out of a 3 year relationship about a month ago - she actually put the 'nail' in the coffin so to speak via TEXT on the day of our 3 year anniversary. Strange thing was we never fought, she just felt some "void" between us; wondering if she got cold feet as we had begun to seriously discuss marriage, etc.
Hang in there - things will get easier as time passes. You've probably read the advice before here but it holds true: keep yourself busy, distract yourself essentially. If it's meant to be, you guys will work it out. Do your best, do whatever you think that you have to do if you feel she is "the one" so you can look back with no regrets. I'm about 4 weeks in right now and I am LIGHTYEARS better than I was initially after the breakup. Time, unfortunately, is the best medicine. Just do not drive yourself insane by watching your phone hoping for texts and or checking her Facebook. I myself am guilty of this, and *every little change* you'll think has something to do with you or is a message being sent to you (aka - every added friend, every added picture, every profile picture change... everything).
Improve *yourself* during this time. Reunite with friends, make new ones, work out like a crazy man. Improve yourself and when you ultimately feel better you'll either be back with her, or ready/more confident to meet your next mate.