I feel your sadness and your pain. It seems that two broken people are in that downward spiral - spiraling out of control. It is soooo difficult to reverse that spiral. But know that it can be done - with a lot of hard, individual work; both committed 100% to and honest, transparent relationship.
Have you thought about individual counseling (IC) for both of you. Until you are both healthy as individuals, the M can't be healthy.
Is your MC trained/experienced in dealing with infidelity and all that entails to heal? It sounds like everything from the past has just been rug-swept and nothing has been confronted head-on. It's painful and difficult, but the only way to get through this is to work through it.
Welcome to SI. You will find a lot of support here. Check out the healing library (top left corner box). There is a ton of info there that would be helpful to both of you.