In Love With My Best Friend: What Should I Do?

I think I'm in love with my best friend. We have been friends on and off for 5 years. He's been my rock for such a long time and I can't think of anyone I trust more than him. Our families have even made comments about us getting married when we are older. Lately, we have been much closer and we have talked about being together. We tried a few times but he told me a few nights ago that he couldn't handle being in a relationship with me because of the mood shifts I get when I'm in a depressive episode. He says that I'm not myself. I know that I'm different when I'm going through the episodes but it just hurt to hear. He's the only person I actually want to have around to help me. My family says that they are there for me but they normally make it worse. Everything just feels so much better when I'm with him. It feels like my pain goes away whenever he holds me tight. And he understands that we don't always have to talk, we can just sit together and enjoy the company. I'm just not sure what to do.