Yowie. This is some pretty serious stuff you're dealing with, what with your break-up and the relationship between M and H being on the rocks. No wonder your head is spinning.
I can't pretend to know what you should do, but here's what I would do.
I'd take a breather. Give myself some time and space to work on my own issues, and to heal. I would stay in contact with M & H while being careful to give them time and room to work on their issues, individually and as a couple.
Keep in mind, Io55, that thier relationship had issues before you came on the scene. Your arrival may have exacerbated the situation, or even revealed problems that had not yet been identified, but you did not create thier troubles. So give yourself a break on that front.
Good luck to you and everyone involved. And welcome to the forum.