The sad reality, I believe, is that you cannot control your parents' relationship. You can extend your loving concern to each of your parents, but I think you're going to have to set some boundaries with your Mother regarding her venting to you about your Father. This will also save you from being belittled. You will have to be quite firm, but not angry. Try to stick with "I" statements, rather than attack her behaviors--"I cannot allow myself to be put in the middle of your relationship with my Father" rather than "You always put me in the middle of your fights and then attack me."

Let her know repeatedly, if necessary, that you will not discuss, nor be the recipient of, her marriage issues. Make an offer to talk with her about whatever other issue you'd like and express your caring of her. If she refuses, tell her you're going to have to hang up now. And do it.

The goal is to preserve your individual relationship with each parent, but not be dragged into their marriage problems.

You may want to seek some support just for yourself in coming to terms with all of this--but don't expect to fix them.