Hello there.

Having gone undiagnosed for most of my life (just last month I received the official diagnosis along with some medication), I can tell you that my states have affected my marriage. Your fiancee sounds very great in that he wants to be with you and exercises great patience when you escalate, but over time this can wear on a person, no matter the strength they have in knowing your daily, lifetime struggle with Bipolar disorder. This isn't to say it can't or won't work; my husband and I have found renewed hope for ourselves, but it's something we have to work on every single day, and for me, it's a simultaneous effort because I'm working on my mental illness in the process.
Are you in therapy at the moment? That was one of the best things that happened to me, was seeing my therapist and taking group therapy, too. I can always confide in my husband, but when you are able to find a comfortable, positive outlet outside of your relationship, I find you're best able to grow and heal (individually and as a couple). I've said and done some horrible things to my husband, so you're not alone there, but be advised that he might not always be so valiant in his effort down the road to brush it off. People do reach breaking points, even when their significant other has a diagnosed illness of which they are aware; they're only human, of course, just like we're only humans with a mental illness we're trying to level out.
I wish all the best for you and your fiancee, and hope that together, you guys will find the best option suitable to your needs and relationship.