I feel your pain right now. I just joined this page because I'm going through the exact same thing. I accidentally logged into my fiance/girlfriend's messages and found out she has been carrying on a romance and was kissing a guy there. Her texts got sexual in nature but didn't prove they actually did anything, just talk. She fessed up to kissing but then this guy's girlfriend (who's married) started texting my girlfriend to stay away from him. Yeah, real Jerry Springer stuff. She promised me it was just a kiss and they're just good friends with a connection, but I later found out the staff had to separate them because they were getting too close. Then I found out they both colluded to lie to me about his departure date and he's really still there. He actually called me to try to "smooth things over," and confessed they both came up with the story of him leaving so I'd feel better. The lies have been constant for about a week now and I'm an emotional wreck.
I'm not saying that's what's going on in your situation. But I know the anxiety and fear you're going through. I've been with this woman for four years and have put up with so much hoping she'd get healthy. She's there for 45 days and now has a "special friend." What is this, high school summer camp? When do these people come back to reality and to the people that actually love them?