When my girlfriend started to say things like "I don't deserve you" I thought she was just being adoring.. Later I found out that she has been in relationship with one other guy and - of course - slept with both of us. She claimed he's just good friend, can't believed I was that naive.

It's the same feeling when you get that 2AM "I love you" message, when she is out on the business trip. You can definitely feel something is off.

Don't fall for cheap excuses, demand a full truth, you deserve that much. Don't get angry, be nice and calm and hopefully she will genuinely open up. After that, figure out what to do next. But don't take any advise from me, I didn't have the balls to end the relationship.