Thanks for the thoughtful replies. It gives me warm fuzzies to have this kind of community available, and I feel better for having gotten some feedback.
I think this must be about change, as a couple of people suggested. RedPepper, you're right that I'm really happy with the dynamic between my two partners and me, and so I'm reluctant to embrace anything that might negatively impact that. I'm going to try to just be with this feeling and let the whole situation play out. I really am happy for A, so maybe reminding myself that change is an inevitable part of life will help me focus more on the happy feelings.