<human>: Can you condense the text into three sentences or less?

So I've been seeing this girl for 6 months now, and things are going great. When we started dating she told me about this friend of hers who she met last year and went on a couple of dates with. Now I'm always very sceptical about girls who have guys as their best friend, but she assured me that they were just friends and that they just stopped seeing each other because it didn't work out.

I subsequently got concrete evidence that this is a lie. She used to write on her Twitter and blog about how much she loved this guy when they were dating, and she had been sleeping with him during the Summer. The reason they stopped dating is because he didn't want her, yet she always talks about him. She swore to my face that she hadn't slept with him when I asked her 6 months ago, even though I asked in a very reasonable and non-accusatory way.

I'm very worried about this. She speaks to this guy virtually every day by text, and has gone out with him for drinks alone on at least one occasion that I know about.

I honestly don't think she would cheat on me with this guy, but I feel betrayed on an emotional level. I might be old fashioned, but I personally think you shouldn't stay close friends with someone you used to have a sexual relationship with when you have a new BF or GF, and had I known the truth from the beginning I wouldn't have entered into this relationship.

I don't know how to bring this up, but I can't go on like this. The only thing that would make me feel better is if she stopped hanging out and talking with this guy, but I don't want to be controlling and obviously can't make her do something she doesn't want to.

I'd appreciate any advice guys, I love her very much but I can't help fell like I'm being made a fool of.
<bot>: My GF is best friends with a guy she used to be (and perhaps still is) in love with. Help!