Trust your instincts. It sounds like he’s having an affair with his ex-girlfriend. Since you’ve been together for 12 years and have a 4 year old child together, you have a lot invested. The important thing to remember is that cheating doesn’t happen in a vacuum. It’s very easy to point fingers and blame someone, but this isn’t a person who is a chronic liar. He’s someone who decided to go outside your relationship to feel good about himself. The question for you is, what was he getting from his extracurricular romance that he wasn’t getting at home, and if you can answer that, you’ll find a way to heal the relationship. Sexual betrayal is very devastating, but it’s not always the end of the relationship. Open communication and talk to him about why he went to her and what he gets there… and then, if he wants to make things work with you, figure out where the two of you can begin to heal. 😉 I hope that helps. Let me know how things go.