Have you tried to explain to your dad that treatment for a medical problem is a decision that adults make for themselves? That you've discussed your options with professionals & you're very comfortable with your decision?

I'm so proud of you for doing the right thing about your drinking. You seem to have finally seen that as the mistake it was and made the mature choice to eliminate that option.

I'm assuming BTW you're withholding nothing from your fiancée--right?

The side effects of lithium make it ill advised for many of us if there is another way. You have to be thoroughly honest with yourself about how well your plan to control your symptoms is working. But even if down the road you were to decide that you did need some drug support, I can't see why lithium would come into the picture unless so many other meds had proved unsatisfactory--a situation highly unlikely.

How does your mom feel about all this? She's the one you will ultimately be called on to show support for. Does she have faith in you?