Most Helpful Guy
You should be asking the question why do you allow him to walk all over you and why in the holy fuck would you want this jerk back into your life to use you and abuse your feelings like he does
Change? No f-ing way. People like that do not change often, and if they do, it takes a long time for them to evaluate their horrible behavior and accept they have to change for their benefit. You cannot change this guy, and as long as you are making it clear to him, there are no consequences to him shitting all over you, he won't care and will continue to screw who he wants and blatantly flirt with other women.
Seriously? For your sake, get rid of this guy and start working on your self-esteem, because it's really low right now and you're going to continue to attract this kind of crap until you realize you need to love and respect yourself or other people won't.
I'm not making fun of you or trying to be mean, but you asked our opinion, and I'm telling you mine. I think you're a really good person, but there's something in your "picker" that's broken. One guy doesn't always define who you are, but his repeated pattern of selfishness and your tolerance for that suggests you need to work on yourself. Put boundaries up (not walls) that you will abide by and expect future partners to respect. Otherwise you'll continue on this trend of unhappy relationships.