Dumped for MIL, you need to be the one who is set free. I spent many years as a "victim" in a M where my H chose his mother over me. I can tell from what you say in your post that you can think of nothing else but your stitch. It is poison to you.
When a woman is M to a "mamma's boy" it will be a miserable life. You have no children. If you did, the MIL would take more control as she would dictate to the son how they should be raised and she would try to turn the children against you (if she's anything like mine was). Don't have kids with this man thinking it would draw him to you and away from his mother.
I'm sorry that I can't give any hope. I think you need to be free of this man and his mother.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!