I can't get over being cheated on...?

I dated my girlfriend for three years, and much of the time it was long distance. I love her and for the last year she always "joked" about liking this guy at her work. When I saw them together they always flirted, had that little glint in thier eyes, and I could tell she liked him. I thought she was always faithful, but now I am doubtful. This guy she always flirted with, let him smack her ***, gave eachother back rubs, etc. Then she broke up with me for the reason that "my career choice wasn't what she wanted in a guy". Then, the NEXT day they are kissing and "dating". I feel like it has gone on longer than this (when I was away). I have always been 100% faithfull and know I can do better but I cant get them together out of my head. I'm hurting, and I'd really appreciate some advice. =/