How do I get my parents to understand and let me move out?

I'm 21 and taking a break from college. I met the love of my life (he's 25) and we are going to get engaged with a year or two. However, I do have medical problems like bipolar and my parents don't think I'm ready to be out on my own at all. The love of my life has seen me through my ups and downs and he has his own place that we can both afford. We want to move in together but I'm afraid my parents will be resistant. They are old fashioned and don't think people should live together until they are married. Because I'm bipolar they don't think I'm ready to be an adult by myself. Help me out here... how do I break it to my parents that I'm leaving? I love my mom, don't have a good relationship with my dad, and don't want to become estranged from them.