Why do i have feelings for my ex?

well me and my ex broke up 4 years ago we only dated for 5 months he cheated on me but he told me the next day he was like ill be hones with u so he told me i cried but forgave hem he never told me that he cheated on me untilll when we broke up i was in love well i moved back home to missouri after collage we seen each other once in those 4 years and thinks happend between us.. well after that we talked every month then we will stop and year later or six months later we will talk on aim again and everytime he asks for my number and says i wanna see u when i come to missouri he does once a moth then we stop but he finds me on facebook myspace he will friend request me and i will block hem because i have a boyfriend but i have feelings for hem well we havent talked in 7 months i found hem on myspace and wrote hem a message saying hi how r u things like that he replys back ( SAYING HI HOW ARE U ITS ABOUT TIME U TOUGHT ABOUT ME CAN I GET UR NUMBER WHAT DOES THAT MEAN I DONT GET GUYS SOMETIMES) i havent replyed back yet my been thinking about hem all day but the problem is i cant give hem my number i have a boyfriend and we been together for 2 years we moved in together but my ex is always back of my mind and i dont have feeling for my boyfriend like i dd for my ex and my heart and mind is telling me i should be with my ex i cry sometimes when i think about my ex i love hem and he always tells me when i told hem about the probles with my boyfriend he will tell me u dont need he, find someone who will trat u better ur better then that soo i dont know what to do and it been 4 years what should i do pleas help