Why can't I get over my lying, cheating ex?

This is weird for me asking strangers for advice, but a co-worker seemed to think it might help, so here goes...

After flirting with and going out a few times with my personal trainer, we started seeing each other (he's 13 years younger than me). It lasted about 6 months. Towards the end, he became more and more distant until he sent a text one day saying he just wanted to be friends. I asked him if he was seeing other people and he said no - he only wanted to slow things down with us because he was busy. Despite a few red flags, I took him at his word.

A few weeks later, he admitted he went out on a few dates with a girl who he later told me was "making him a better man," and that she got to him like no other (he was in love, could see himself marrying her, etc). He said that he was slow to admit he started seeing this girl because he didn't know where it was headed and he didn't want to hurt me.

A few weeks after that, he admitted that someone from his past he described as "just a friend" was in fact someone he had been dating just prior to us, but he claimed they never slept together. About a month later, he admitted they slept together but his excuse for lying was that he was trying to stay best friends with her and he didn't want me to feel awkward about that.

Fast forward 3 months...he's now been my roommate (platonic) for 6 1/2 weeks (his "special" girl who works abroad in Ethiopia has no clue). He admitted to me just last night that he had a one night stand with one of his other trainer's clients a week after he last slept with me (but long before he "friended" me) and that he had also gone out on dates with another girl between me and this "love of his life" (again, this was all weeks before he "friended" me). He expects me to believe that he only went on one date with this other girl and that he never took her seriously, but oddly enough, he told me she's been texting him a lot lately and dropped by at his gym yesterday.

With all that, here's my question...why do I still love this jerk? Why do I still want him to SEE me? I've never lacked confidence, independence, and I have my sh*t together, but I don't understand why I allow such a manipulative liar stay in my life?

[sidenote: he and his "special" girl blame me for a series of weird e-mails and web postings. so not my problem! Despite this, he still takes advantage of my offers of friendship (he's called last minute for rides from the airport and other favors). He said she is sure I'm the "bad guy" - probably based on what he's told her since I've never met the girl - and has considered taking legal action. NOT COOL to falsely accuse anyone of anything!. I want to confront her and report her ass to the police, but he's yelled at me to not talk to her...seriously? What to do?]
Why can't I get over my lying, cheating ex?
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