How can i forget someone who says still likes me but has a girlfriend?

this is the problem. The guy i used to talk to did something bad and i was so mad and sad at the same time didnt wanna talk to him. anyways later on he said he wanted to still talk to me and i never gave him an answer until later when i was about to i found out that he likes me and another girl. he told me that he liked both of us. but after a week he or she ask him out an thy started going out. he seems shocked that i actually supported him nd said that is good he is going with someone and that she was pretty even tho she aint. lol anyways im so thru with him because he is a biiiig flirtacious ith girls and stuff and now he has a gf he flirts more i guess. i dont know bt he still talks to me like he used to but he seems like he really like her which is good for him. but i need hel to stop liking him so any sugestions?