Yes I do understand that completely and have for a long time. That situation was not my choice and I was against it from the beginning. When I became stubborn was she went from asking to demanding. I felt as though she was trying to control me and tell me what to do. Pride? Yes! Ego? Yes! Wrong? Absolutely!!! Question is what now? Once you make the mistake can you change it? Can you fix it?
Depends upon what you are referring to as the mistake. Do you mean by not asking your mother to leave when your W "demanded"?
Since I went through some of the same thing, I can tell you that no matter how close the MIL and DIL are before they live together...they will never have the same R again. My parents tried to warn me, but I just could not imagine my MIL being anything like she turned out to be. And, they were right, we were never the same again.
I believe with the W and her MIL....they are in some way trying to be the man's fist choice. They are competitive for his time and attention. If he doesn't do what his W wants and follows his mom's advice, then it hurts the W. I don't know how to explain it well. Before M, his mother is the only woman in his life. She's used to telling him things and doing things for him. But, when he gets M, then it's the other woman's right to do that. Some are very jealous of each other. Either way, it is usually a sign of lack of confidence and where there place is with that man. With my MIL, she meant to be the queen bee and rule her grown children....and the grandchildren. I was determined that she wasn't. So....you can guess how lovely those years were for everyone!
I'm sure your mother is fine, but your W did not feel that she was first place in your heart and life. That hurt me until the day my MIL died. I'm not sure my H ever understood, even though I had tried for years to tell him how I felt.
The two of you are still young and have time to make amends. I don't know it you've seen her thread, but there was a poster who was called "Dumped for MIL".
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!