Going thru a similar situation myself. That's how I wound up on this thread.
Found out my old lady with "talking" to another guy who happened to be a friend of hers and a semi-acquaintance of mine (never really liked him, kind of a scumbag). Talking all day and all night (she works midnight turn) Apparantly, he was alot closer to her than me. They've been "talking" since February and she had always insisted that they weren't doing anything else. And I can tell you, 99% of the people she talks to on her cell phone (which is never more than an arm's length away from her) are men. I've always had a problem with it and she knows it. "Don't worry. They're only friends." Anyhow, I never believed her that nothing was going on and she eventually tells me that she was indeed sleeping with the guy. Problem I have is I have a baby to worry about, also.
Take some advice buddy, ditch the bitch before you're in over your head. I know it's hard to let go, but you gotta do it.