What should I do if my mom is having an emotional breakdown?

I'm a graduate student and I live with my parents. I am engaged to a man who is currently in school to be a pilot, to be completed at the end of this year, and we have been engaged for over a year. We have not started planning our wedding yet though because every time I start talking about it, my mom has a breakdown. My dad loves my fiance and has no problem with our engagement, but sometimes I'll come home from work or school and my mom will be waiting for me to tell me she is leaving my dad because he supports my engagement. She cried for a week once because my fiance was trying to make me feel better about a grade that I was worried about, and she decided that it was all his fault and he was a bad influence on me and wanted me to fail in life. She says he lacks ambition and will never amount to anything, when he is a top student in his program. Her childish behavior is nothing new, as she frequently packed her bags and "left" my dad numerous times throughout my childhood over things that seem silly, and then came back a few hours later. Everything sends her into an emotional panic and there is no rest in our house. She really has a hold over our family, and we all walk on eggshells to keep her happy. The thing is, I love her and she loves me and she isn't necessarily a bad mother, but I feel strangled. At the same time, I do not want to hurt her because I can't stand it when she disapproves of me. I am terrified to even mention my relationship around her anymore. Everyone else's engagement is a happy time, and mine might as well be a cancer diagnosis. I want to reach out to someone, like another family member, but I don't know how appropriate it would be to talk badly about my mom to her family. I need to move out I suppose, but I can't afford it at the moment. Is she crazy? What should I do?