Me and my boyfriend have been seeing each other for 1 1/2 yrs. He always been funny and sweet and has never given me a reason to think that he'd cheat on me. Since we've been together he and my best friend (who has a history of sleeping around)became good friends, which didn't bother me since normally she hates the guys I date. But about 6 months ago I started getting the feeling she had feelings towards him. I didn't get too worked up because I knew he didn't think of her that way, often calling her his "brother". But the longer it went on the more it began to get to me. They would hang out while I was at work then come get me and we all would hang out then he and I would go home. That time that he spent with her that I wasn't there I had this sick feeling in my stomach. She and I worked in the same place and one day when I was going through our system I found her account and saw that she had him listed under "husband". This bothered the about me, that's the first time I asked him if there was something going on between them. He said no and that it was probably just a joke she was playing knowing that I would find it. I told him I was sure she was in love with him, but he said I was just being paranoid and that he loved me and has never given me a reason to think he would hurt me. Well they stayed friends and I kept saying that I didn't trust her. Well about a month ago I was at a friends birthday party. We were all sitting around drinking and having a good time when one of my good friends pulls me aside and tells me that my best friend had told him she had been having guy problems, that she had been seeing my boyfriend and that they had just a few nights before that had sex. I was furious and pulled my boyfriend into a bedroom and asked him about it. At first he thought I was kidding or lying, so I had to have the friend who told me come in and tell him to his face. My boyfriend denied it and said that he was through and that he would no longer hang out with her without other people there and that he wouldn't talk to her as often. But that he couldn't just cut her off because she was a good friend and he didn't totally believe that she had said that. Well that solved things for about 2 weeks. I then found out that he and his friend had hung out with her and he never mentioned it to me. I asked him why he didn't tell me and he said he didn't want me to get upset, and that someone else was there so he hadn't "broken any rules" I let it go and moved on. But then yesterday I found a bunch of blogs of hers online talking about this guy. They said stuff like " i love the way you hold me and say that i make you happy but i know you're lying" and then there was a blog that was directed at a female that said stuff like " i know you're miserable, your life is falling apart dont lie to me about how happy you are . i know its not true. dont try to make me jealous of what you want me to think you guys have" I asked her about them and she said she didn't remember what they were about. That night I asked my boyfriend if she ever emailed him. He said they had one stream of emails and that I could read it if I didn't believe him. So I did. They were all talking about they're favorite video game until the last one that was from her that said " im done i dont like feelng like i'm being lied to" I asked him about it and he said he had no idea what that meant and that's why the conversation stopped there. Then I made a mental note of the date she wrote that. I looked up her blogs and sure enough there was a blog from that same day that was talking about "i love you but i'm done with this , you dont really care about me, and i dont like being lied to" so I sat him down and asked him again if he knew what the blogs were about he said no, I asked him if he had cheated on me he said no. I don't know whether I should believe him. He promised not to see or talk to her at all anymore. And he hasn't changed at all behavior wise. He's still sweet and caring and I feel like I should trust him, while she has a history of lying. But its such a big lie. I don't know what to think.

He may have been with her for awhile and afraid of losing you if he tells you the truth.
You can either just trust that they are through with whatever did or did not happen
And just accept where things are now
Or
You can keep having mistrust for him and drive him away.
He must have decided that you mean more to him than any mistake he may or may not have made with her.
Until he gives you any further reason to not trust him give him the benefit of the doubt.

I don't know what to think.
You believe your man, and not you lying girlfriend, until evidence says otherwise.

I've been in a situation before where I cheated on a girl I was with.. she was also suspicious but I continually denied it.. reason being I realized how much I cared about her and I didn't want to lose her over a stupid mistake (even though I would have deserved losing her)..
You're in a difficult situation.. whatever the case I feel that you have to sort it out.. Try and talk to him and demand the truth.. If he insists that he didn't cheat on you then just stop bothering him about it and continue treading carefully.. Don't be paranoid to an extent that it will ruin your relationship - because maybe he didn't cheat.. If you see that another hint arises then start to worry about it..
It will put a lot of pressure on your relationship if you continually doubt whether he's telling the truth or not and it will eventually tear you apart..
I was guilty of cheating on her and I was continually interrogated on whether I cheated or not and even though I hated lying to her and I was a right a**hole for cheating on her - I also felt really pressured and cornered and somewhat tired of her interrogating me all the time.
I hope this helps