Wait, he doesn't understand what bipolar means or he doesn't understand because you've never told him?

If it's the latter, I've found that all the people closest to me have said, "You're still you," and have acted as if it didn't really matter to them. In a way, they're right. The bipolar has not affected my kindness, compassion, being a hard worker, diligent, responsible, etc. You are who you are before the dx as after.

So why bother with the dx or telling anyone? Your brain chemistry will mess with you from time to time and you'll feel terrible depression if not wonderful elation followed by depression again (if untreated with meds and therapy, diet & exercise). It's like any disease, it's just one in your brain. You will go through neutral periods as well with no symptoms. You might just feel plain old good. But on other days you may rapid cycle through a half dozen emotions and wear yourself out.

If your spouse to be understands all this, he can know how better to relate to you and to support you in the seeking of a medication or group of medications (usually) that is right for you. This will curb the lowest lows and highest highs. The medication merry go round is hard to go through without support and love. Some of them will have terrible side effects and you'll have to switch to another and another. It's been 14 months for me since I've been looking for a good combo, but then, I didn't get dx'd until June of this year, so they were probably giving me all the wrong stuff!

Anyway, I think I may have found the right combo. I'm starting to get moments of euthymia...a sense of well being.
I'm on:
Lamictal: mood stabilizer
Abilify: atypical antipsychotic/anti-depressant
Wellbutrin: anti-depressant

And I have sleep aids. I've had bad insomnia for weeks because I've been manic for over a month.

My only advice to you is to read up and know as much as possible as you can about the disease, print out material, and prepare yourself for talking with your fiance about it. Don't let the fears or lack of info push you apart. This disease is fully manageable like diabetes. He needs to know all of you just as you would want to know as much as possible about him. Why? If after you know it all you still love each other, how much stronger does that prove your caring about each other?

I worried about telling anyone in the beginning too...and only those closest to me know about it. I always assumed they'd think I'd be wild and breaking stuff when manic. But everyone's mania suits their personality. Mind is more like a happy drunk. Rambling fast speech, telling jokes, lack of inhibition, etc. Some people do get violent. I think you know by now who you are. And I think if he really loves you, after he gets all the info, including treatments available, he will stay with you. Hang in there.