Why cant I let him go!?

Ok I am with a lovely guy, very caring, kind , has a few possesive issues, but hey no ones perfect... anyway curiousity got the better of me and i have had a casual fling over a year which a much older guy, i think i fell in love with him at once stage, but it is clear he is a loser and a jerk (he was married, no longer now tho as he got snapped with ANOTHER mitress) and was only ever using me (ignored me unless he thought he was going to get some , and totally disregards my feelings - i think he is emotionaly deprived lol) anyway despite all this i just can't seem to forget him and let him go, i finally cracked tonight and told him he is a jerk over txt but he just ignores me and his indifference drives me crazy! why can't i let it go!