I'm not in the same boat, but . . .
It is commanded that we have to honor and respect our parents. No ifs, ands, or buts. There are no exceptions.
Hashem is all too aware that parents aren't perfect, but He provided no caveat. We have to honor and respect our parents. End of story.
I really feel for the letter-writer, I do. But I have found that sometimes situations call for a flip in perspective. Yes, her mother is a shlump and people can and will potentially hold that against the daughter. But doesn't the same Hashem that gave us parents and the Aseres HaDibros the same Hashem that makes shidduchim?
I happen to know of a family where the mother isn't as . . . polished as some. Her children are good kids and always respect her. You know what? All of her children got married, and not only that, they made very good shidduchim.
Instead of focusing on the mother as the problem, the daughter can flip the perspective: "My job is to respect my mother. That I can control. I cannot control my dating life: That I leave to Hashem. I have my job, and Hashem has His."
This is a perfect opportunity to grow not only at being the best daughter possible, but also to grow in emunah and bitachon. It is not easy. But it is not meant to be. What is worth having is worth working hard for.