How can I learn to trust my boyfriend again?

We have been dating for about 9 months (first of july to present); we did break up for about a week in November because he said he needed to figure things out in his life and that he felt we would make better friends but really it was because of her (his ex girlfriend that he dated for like three years). I can remember saying goodbye to him one night after we broke up and his phone was ringing…and he didn’t answer it..but he always answers his phone..it didn’t clue in then but it was her calling him. Then a couple days after that night, it was a wednesday night, he called me around 1 am and asked me to come get him as he was running down the street from her who was chasing him. Me being me, I went to get him…he said he made a big mistake ever trying to get back with her…by the way she drinks every night…is the biggest slut you will ever meet, cheated on him so many times, and they never got along…and bad fights..like burning clothes and shit.
My boyfriend and I get along great most of the time…except when we drink..that always leads to problems because he likes to flurt when is drinking and I don’t put up with it. One night we got into a fight at the bar, we both said we wanted to break up and called each other some names. Then when I was getting my coat who did I see him standing next to at the bar, his ex who he knows how much I hate (you see she has been trying to break us up for the longest time and writes and texts me lies just to get me going, and she says she doesn’t want to get back together with him, that she just want to get back at him or something).
I flipped out. Anyways he ends up going to a party with her and her friends, he says to get back at me..her friends start calling my cell saying how him and his ex are having sex in the bedroom…after this little episode I was like bipolar for at least 2 1/2 weeks…we talked and he said he didn’t do anything with her and that they were at a party and they stole his phone and he said he spent then night at a friends..did he really? or did they have sex? We didn’t break up, but very, very close..he blocked her from msn, facebook along with all her friends and says he only loves me and that he hates her.
I want to trust him..and when we first met I had no trouble (cause he never did anything, or I didn’t catch him). How can I trust him again? I love him so much and he says the same about me…we laugh all the time but exs along with money (but thats another issue) are two major problems for us…how can I deal with these feelings that he is going to cheat on me. Every time he goes anywheres all I can think of is that he is with her…what do i do???