I can't move on

I'm a 21 year old girl & here's the story: 3 years ago I went to a play at the college I was going to attend. There was a boy in the cast who I thought was pretty cute. Let's call him Rob. Flash forward a year later, & Rob & myself were both involved in our college's theatre department. I mostly knew him by reputation & decided that I didn't like him, although I was secretly attracted to him. In the months that passed he started dating a friend of mine & I'd often was a third wheel. I got to know Rob & I realized that despite his outward brashness, we were very alike. However, my friend & him broke up; it was very messy & she moved away. After this I realized that I was in love with him & was working up the nerve to ask him out. But before I could do this, he started dating another girl & they fell madly in love. It completely crushed me but I told no one. After 2 years at school I decided to quit (for unrelated reasons) & although I didn't see Rob anymore I couldn't stop thinking about him. This is completely unlike me, I get over these things pretty quickly, but this was different. So I decided to tell him how I felt, & naturally he didn't feel the same way as he and his girlfriend are still head over heels for each other. I thought I could finally move on, but I still find myself constantly crying over him. We never dated but this is 100 times more difficult than any break up I've had. I've never felt like this before. I can't move on & I don't know what to do.