You need to step back and take a long careful look at the complex dynamic you are at the centre of. It seems you are in a tug of war with regards to time and emotional energy with your DM and your marriage.
Currently your DM has won and your DH has dropped the rope.
You are torn, exhausted, depleted and not where you want to be.
The most important people in this system who have not been mentioned are your young DCs. Are they your priority?
It seems for all of their young lives you have given a lot of your finite emotional and physical energy to your parents. Was your NPD DM dictating the terms, manipulating and monopolising your time and energy around your DF’s care for the last 12 years as well?
This is when your marriage and family life began to erode. Your DCs have been short changed. And as they enter teenage years they will be short changed further as your NPD DM increases her demands on you.
What do you want for their future?
Who are you going to prioritise?
How are you going to choose to allocate your finite time, emotional and physical energy?
YOU are the only who can decide how this plays out from now on.
Which ever way your marriage goes your relationship with your DM is the issue and your DCs (and your own MH) are the collateral damage.
It doesn’t have to be this way.
That’s the one dynamic that you can