Yeah, I'm going to third what Nurse and Chaotic said. A year-long affair is not mania. Perhaps it was originally a manic episode when you started but continuing to do it is because you're, in fact, cheating willingly.

You fully know that what you're doing is wrong. Bipolar Disorder isn't an excuse for what you do wrong. We all have to accept our mistakes.

Sorry but you need to tell your husband and either leave him or stop the affair immediately. It is cheating, a betrayal of trust and is at someone else's expense. He does not deserve that, and there is nobody else to blame except yourself and perhaps the person you're having an affair with.

To me, it sounds like a relationship issue and not a psychiatric illness. That's from what you've said. It really doesn't look as though you nearly match the diagnostic criteria for a Bipolar diagnosis going from what you've written, and that's my unprofessional advice. None of us can diagnose you and neither can we excuse your behaviour for you.