Sorry and I'm back for a third post..
On the masochism thing. You might not consider yourself a masochist emotionally speaking because you are not 'enjoying' this emotional shitfest. However it is possible to be addicted to drama and emotionally masochistic type relationships where you suffer a lot but seem unable to get yourself out of it.
People usually get like this because this is how they are used to interpersonal relationships being and they don't know how else to be. They also attract people to them who love the drama and will play it out with them..like this MM who clearly is more than happy to fuck with his wife and you (and god knows who else) whilst moaning about how awful it all is. Essentially you are both feeding into each others compulsion for dramatics and damaging interactions, not necessarily because you planned it that way, but because you haven't got to the root of why you choose to be around people like this and why you don't feel attracted to a stable, solid and healthy relationship.
I would bet my current bank balance you have issues with being bored and needing excitement and adrenalin in your life. Maybe you have had stress-fuelled childhood.
Anyway whatever it is it is possible to break out of the cycle but you have to be really honest with yourself and understand you are making a choice to continue having damaging interactions and the only way that will change is when you look at yourself and decide you want to find your self-worth. You can't magic it up out of nowhere, you have to work for it. Small things every day, and most importantly setting the intention that you will work for it.
Because if you don't then when eventually this does end completely, either by him cutting you out or you finding someone else..the same sort of thing will happen again. Your problems won't end just by him going. They will only be sorted when you make the decision to change and to start to learn how to give a shit about yourself.
I do feel for you because I think that often people don't understand that this sort of damaging behaviour is learned behaviour and really tough to break out of. But you have to start really focusing on yourself and learning to understand yourself and how to care for yourself. Or you will remain stuck.
I know this might be a lot to take in, but I have been in similar places myself, making bad choices and getting hurt, over and over again, and feeling shit about myself for not being able to break out of it. I know how hard it sucks. I also doubt you get much enjoyment out of it, but you do get something out of it and a compulsion to have drama is not necessarily about being a stereotypical 'drama queen' and often about shitty boundaries, low self-esteem and compulsive / addictive behaviour.
I am going to link a poem which I almost never do on places like here because it sums this whole concept of making the choice up perfectly. Having gone from similar to where you are at present to now being a happy woman who values herself and makes choices based on her own wellbeing first and foremost, (which then did lead me into a loving healthy relationship) I can tell you it takes work, balls and a lot of long hard honest looks in the mirror.
It is 100% worth it though and can be done. You can do it.
The Journey
One day you finally knew
what you had to do, and began,
though the voices around you
kept shouting
their bad advice --
though the whole house
began to tremble
and you felt the old tug
at your ankles.
"Mend my life!"
each voice cried.
But you didn't stop.
You knew what you had to do,
though the wind pried
with its stiff fingers
at the very foundations,
though their melancholy
was terrible.
It was already late
enough, and a wild night,
and the road full of fallen
branches and stones.
But little by little,
as you left their voices behind,
the stars began to burn
through the sheets of clouds,
and there was a new voice
which you slowly
recognized as your own,
that kept you company
as you strode deeper and deeper
into the world,
determined to do
the only thing you could do --
determined to save
the only life you could save.