I am in the exact same situation. ALmost EXACTLY as you decribe with feeling unloved and unappreciated, the wife hating the bipolar, etc.
My wife and kids left the country for 4 weeks. I DID cheat, and it was super fun. I also fell in love with the chick. My wife and I are getting divorced now. SHe doesn;t even know I cheated, I just exploded on her because when she came home she was verbbally abusive to me, and compared to the new woman, she seemed awful. Divorce sucks, I tried to get her back, but it is too late. Oh well, no regrets, WHY STAY IN A MARRIAGE WITH NO LOVE??? Find someone else, or just go have fun dating.
Just kidding, do whatever you want. I just accepted that my marriage was crap, and I am flirting and dating girls already. It is fun.

The part aboutt eh kid sucks. I don't get to see my kids much now. But I still see them and it is cool. THE ONLY REASON I tried to get my wife back, was because of the kids. Everyone including my therapist, says DON'T stay in a loveless marriage for the children...big mistake.