Why cant i let go?

Me and my ex have been broken up a little over a year.It was a painful break up for me and my children.I had really thought that I had feel in LOVE for the first time in my life. the realtionship was breath taking But the time came and it ended. and it only ended cause is baby momma held his child over his head.stayin that she would take him away if he didnt go back to her. So he left and they moved to another city.He told me he was sorry but he has to try and make it work so she doesnt take his child away. It really hurt me but i expected it.But i feel as if im going crazy cause i think of him so much.it like i feel his presents.when i go to stores we use to go to together i rethink of every moment we had there. when i lay down at nite i think of the nites we shared. when i cant listen to music cause i will start to cry i can just think of him and my heart beats out of control...Please someone tell me whats wrong