You are so young. The fact is, he has has some sort of connection with another that a married man ought not have. My best advice would be to go.
I have seen this go both ways. But I can tell you that I have lived with that fear myself. It is really not a good way to live, always afraid the other is with someone else, knowing damn well he could be.
Yes, it is.
You should kick him out of the house and let him move in with the woman.
Don't stay in the relationship as you described it. He has to find out the grass is not greener on the other side--on his own! (you can't force-feed him grass).
leave...and lesson learned...
sounds like the typical husband wasn't getting what he needed at home and found it some where else, if he was getting what he needed neither you or him would be on this situation