My fiance is bipolar and my mother won't accept him!?

My fiance and I have been together for four years and my mother has given both of us her disapproval since day one when she found out he had bipolar disorder. We have lived together for three years and he has only had one major mood swing that has required a med change due to off levels. On a day to day basis, he has occasional downs with nothing even noticeable to most. He works full time, is starting school for forensics or veterinary medicine in the fall and volunteers at an animal hospital on the weekends. Most think he is a stand up guy and I'm lucky to have such a man. My mom disapproves because my father and her split up right around the time we met (my father was an alcoholic for 30 years before hand and they're marriage was abusive in the first place). He is always polite to my parents and when we had to stay here for a week (while our place was being worked on) he made dinner, washed dishes, mowed the lawn, took my mom out shopping, etc... Keep in mind he is 23 and I think he's doing great for his age! She believes he is destroying her life by dating me because she can't stand to see me with someone who is a "psychologically ill man." Also keep in mind that she suffers with depression and my father was an alcoholic. You wouldn't even know he's bipolar! He doesn't drink, smoke, do drugs, etc... He works, studies, volunteers, spends time with me, and plays video games! A normal 23 year old! Why is he not good enough for my mother?!