why can't I move on? I have every tool.

Here goes another break up story. Ima try and keep it short and detailed.
5 years together(moved in together after 2.5) I messed up first by talking to some guy(we are 15-16) in the beginning. Learned my lesson I messed up something new and pure over someone meaningless(crazy how even talking to the other guy I didn't want anything serious, I wanted my guy) I was stupid I acknowledge that, a mistake that cost me more then I could handle, but it was to late he was done.
He moved on with a chick who's younger but has been around. (On the other hand I had a Christian childhood, very strict moral parents) and honestly I feel more then struck. He is my first in everything. As was I to him.
Just the tip of the iceberg, but he has done everything with this chick, and I had a one night stand thinking revenge is sweet.
Wrongo.
Made it much worse, he fell for her.
Year and a half later we are friends and they still talk, I still got my feelings but I'm keeping mellow.
But I'll be blunt I miss him when we don't talk or see each other, it's just a mutual thing, or at least it feels that way.
                                  Why can't I move on....