It sounds like he is still in High School: 500+ text messages a day, hour long phone calls, matching tattoos, .... I mean, really!? It also sounds like he has carried a torch for this girl the whole time, and when her H died, he was there to comfort her, which lead to his (maybe) life-long desire of a weekend of sex with her in a cabin.
I was engaged to a girl who broke off that relationship. I was devastated to say the least. And to boot, there was no reason why. I've had to guess and speculate for decades why she did it. BUT, I decided to let her go and let my new fiancé (now WW) into my heart and make a life with her. I made that clean cut. It sounds like he never did.
I also sounds like he is being manipulative: He's fallen into a suicidal depression, he says he's a monster, he doesn't know who he is any more, etc. I am not a counselor by any means, and he may need to get professional help. You can -- and should -- point him in that direction, but he has to want to go there. IMO, however, (and that's all it is - an opinion), he seems to be playing the victim card. He wants you to feel sorry for him. DON'T!
He's maintained contact with his HS sweetheart for all these years, and now when she was at a low point, he was able to move in and be the Knight In Shining Armor (KISA). It seems this is what he always wanted, so let him have it.
And regarding what BeingNaive said:
Also, the matching tattoos? What in the hell is that bullshit?! Talk about a huge slap in your face! Something to permanently connect them and something you would have to see every day? The disrespect is disgusting. I'm so sorry he's doing that to you.
Uh, YEAH! "Amen" to that! He got a souvenir to remember the weekend he finally go to screw his HS sweetheart which you now have to see each time it's exposed?! You get the "blessing" of a visual reminder he committed adultery?! This guy is totally into himself! To make it worse, each time a friend, family member, colleague, or anyone else sees this tattoo, he gets another opportunity to brag about what he did -- cheated on his wife!
I'm sorry to be so long-winded here, but this guy has really struck a nerve with me!
You said you moved out. Maybe, initially, that was the best thing to do. But, talk with a lawyer about that. Leaving the house may give your WH some legal advantage later.
Outta angry one night I said I was done. He just said ok and cried. Next day he made a long term plan to move out. He’s given up on himself and me.
Good! Get him out so he can go be with his tattoo sweetie! And since "he's given up on himself and me," get the D papers rolling right away. You can stop at any time, but maybe this will wake him up from the "dream" he's in and the nightmare he's putting you through.
I've read many times here when the WS leaves for the AP, it rarely works out. If he wasn't faithful to you, what makes you think he will be faithful to her?
Hang in there, girl! This will be tough, but we're here for you. It sounds like this guy needs to run full speed, head first into a lamp post!