My mother has ruined my life and yet I want her to still be a part of it...What should I do?

So let me start this off by saying that I am 20 years old and have watched my mom go from a loving caring mom, to turning the family into a mad house where everyone is afraid to stand up to her and she picks favorites with my sisters. She has a mental condition that she had been prescribed medicine for, but refuses to take her medicine and denies having any problem at all. My grandmother says that it has gone down hill since I was born. She used to leave me alone as a small baby to go on long walks...and it has gone from there....For 4 years now my mother has berated me and belittled me and has said I was a loser who can never make the right choices in life. She has forcibly removed me from relationships when I was dating people in high school and isolated me in my room until I gave up on the idea of being with them. My fiance is the only one who stood up to her and refused to back away and stop dating me just because she said to (just as a side note). In the August of last year (2010), just before I went off to college and unable to take it any longer, I left the house and moved in with her parents, which were more like my mom and dad than my parents were. Naturally, she hates my fiance, who has kept me afloat as I was diagnosed with depression two years ago and received help for it thanks to his nudging reassurance. His only flaw is that he enjoys to drink every now and then (as in once a month) with our friends and he dips. She calls him an alcoholic and a lazy loser for not wanting to have a job while he is attending an auto diesel school to be an auto-mechanic. Which, because of the hands on work that i have witnessed first hand, is like an unpaid job in itself. He only wants to keep me safe and take care of me, but because he is not the one she wants for me she has said horrible things to his face and has spread nasty rumors about us behind our backs to the family. As far as my work ethic in school, I made A's and B's in High School with the exception of two C's and got a 3.85 GPA. My Freshmen year of college I made two C's and got a 3.00 GPA for the year. She has criticized me for switching my major 3 times (two this past college year and once for this upcoming year) and says I am never going to get a degree because I am not dedicated enough to one thing to learn enough to get a good job. I have worked and saved money and got my own car, a used toyota camry which is perfect for me. I am continuing to work and go to school and save for when my fiance and I are going to move in together next spring. Until then, I am staying with my grandparents, who because of the rumors my mom has made don't like the idea of me getting married to him. He doesn't feel like he owes them any explanation as to the way he actually is and feels like once we are married, and he has a job to support me and him that they will see that he is actually worth something. However, the only problem is that none of my family wants to come to the wedding and that upsets me. I have literally stepped back and analyzed why I was with him and looked at his flaws and strengths and made sure I wasn't one of those people who are blind to their partner's bad qualities until they are no longer with them and have had the chance to sober up. I am 100% sure that it will work out and everything will be ok. I just want my old mom back and I want my family to support my decision in marrying my fiance. What should I do?