I really need help on this?

ok i know this is in the wrong category but people never go to the relationships one and i don't know who to go to. for four months over the summer,this guy who is a good friend of mine and I hooked up on and off. we both really liked each other but he didn't want to date anyone so i told him it was over. we didn't talk for a while but then he started dating this girl who is my ex best friend and i really don't like her, but when the started i was kind of forced to be her friend. i was hurt because he said he didn't want to date anyone but it seemed like he was flaunting her in front of me and almost trying to make me jealous. we started being friends again and everything was going great until one weekend when she went away and we started to get close again. we went to the movies and hung out and stuff and i had been putting my feelings away for a long time but i realized that we both still liked each other. he wrote me a letter at the end of the weekend telling me how much i meant to him, it wasn't really romantic but it was sweet and he signed it love so i thought nothing of it, until my best friend told me that it took him 2 hours to write it and it wasn't that long. she also told me that he talked to her and was confused about who or what he wanted, me or the girl he was dating. this was about two weeks ago. in the past week we've hung out more but hes still dating her and i would never hook up with him again unless he was single because i'm not like that. his girlfriend is getting really upset but i don't know what to tell her because she thinks i like him and she says hes uring her that nothing is going on. i'm sick of this. i want to be with him but i don't want to cause drama and we haven't forwardly talked about it yet, but he is very indecisive and his answer to a lot of things is 'i dont know' so i'm scared. WHAT DO I DO???