Hello, I'd like to help you with your question.
It sounds like Mike tries to manipulate the situation to get what he needs. He told you early on when he was still with this wife that he would not cheat, possibly to get your sympathy because his wife was cheating on him. Then he changes gears and wants to be with you before his marriage is over, which you mentioned you felt was cheating. Now after telling you he loves you and having a relationship with you to the point you became his Surety, he gets involved with someone else, leaving you stuck watching over him so he doesn't go to jail.
This situation has hurt you deeply. Getting Mike out of your life is ideal, but since you cannot, you will need to find ways to cope with how he treats you and his behavior around this new woman.
The first thing you can do is set boundaries. As much as possible, keep Mike's things in certain areas of your home, areas that you do not use often. When he is not in your home, take his things and lock them in the room or storage space. That way, you do not see reminders of him all around you and it won't feel as if he is living there.
Also, set boundaries with his behavior. I'm not sure how much control you do have over what he does, but if you can, tell him what you expect from him while he is with you. Limits on internet time, chores, errands or things like that can keep him busy, help you, and keep your relationship on the surface rather than intimate. It changes your role as well.
Try emotionally removing yourself from his life. See him as a temporary inconvenience. Try to block out any thing about his personal life. If he tries to bring up his new relationship, change the subject. Stick to topics having to do with his situation only.
This may have been a rebound relationship for Mike. But it is more likely that Mike has personality issues and uses others. If that is the case, then no relationship he has is going to last long. He will probably not be able to settle with one person.
On the other hand, your feelings were genuine and you were hurt badly by Mike's actions. It may take you some time to heal. That will be difficult when you have to be involved in his life for the next few months. You may want to try maintaining a distance from him emotionally and as much as you can physically until he is out of your life and you can let go and mourn the relationship. Be sure to take care of yourself and allow yourself time to recover. Try removing all reminders of him so it is easier. Talk to friends and family for support and if you feel it's needed, talk to a counselor as well. It will help you let go and move on.
I hope this has helped you,