On my way through years of these mood swings I learned to trust myself in this. It's like there are three main "versions" of myself (depressive, "normal" and manic) who don't believe each other about seriousness of symptoms. So, when I'm living through each phase, I just tell myself: "this is it, what I'm going through now is a real serious problem". Then when my mood changes again, I remember what I told myself before and just trust it.
It's really helping me, I hope it will help someone else to cope, too.