Going through therapy may or may not help you depending on the quality of therapist you end up with....not all therapists are created equal!!!!.
I went through therapy when I was having problems & it became obvious my then husband of 25+ years was a huge part of the problem. It was financial issues & him being an irresponsible immature person still at 54 years old.
I finally just walked out & moved 2100 miles away because he made the divorce process in the mental state I ended up in....IMPOSSIBLE. Finally got divorced 11 years later because of the financial messes he made.
Best thing I ever did was walk out because he was (& never did) going to change. Leaving was the only way I could have ever healed from all I had gone through & I did find a quality therapist in my new location which was a real blessing & I was no longer living with the problem 24/7.
Each situation is different & you need to do what you feel is the right thing. I initially thought that leaving would knock some sense into him & he would change since nothing else ever got through to him....it didn't & after 33 years of being married & living with him, I have never been happier than the day I left. Wishing you the best with this. I know the struggle but only you know what solution will work for you. You have to look out for your own mental wellness first. It is her responsibility to fix what is broken in her because no ine else can