I have tried but cant move on ?

This is all very crazy and i really done understand why i feel ike this ............i was in a realtionship with a guy for three years , we split up abut 4 years ago , hes now with someone else infact married with a child but i still cant get him out of my head why ???????? hes no good i know hes not . although we split up and i know it was very wrong i started seeing him again behind his new womans back, he told me he wasnt happy and missed me. I only founfdout by accident that he is infact married to her now with a child younger than mine and his grandchildren ............why cant i forget this man ????????? wouldnt mind if we were teenagers but far from it we r both mid 40s