Any advice on how my fiance and i can lift our moods so that we feel better.
Attila might well have better advice than me, but from my perspective, I think that I'd start making my own choices now.
You're held hostage with the 'maybe I'll come! Maybe I won't...'
I think you need to make a choice now, simply to take yourself out of that hostage position. You know that your mother will not behave, so given that, do you want her to be there? Do you want to pander to her needs?
Remember, this is your day (the two of you). If all goes well, you only get this once.
She comes or she doesn't - If she's not prepared to step up as your mother on this one, important day, then she's just another wedding guest, and it's really not a big deal if she doesn't show up.
I would definitely stop responding to her, and certainly don't start negotiating with her. She seems to thrive from the drama, and the more you respond, the more she has you dancing on the end of her string.
It's awful, it's horrible when it's sitting in your head, but the only advice I can give is that you try some mindfulness or meditation exercises, focussing on the hear and now. Keep yourself in the moment, and when your mind starts to wander to 'will my mother show up?' stop yourself and think 'yes, that's a troubling thought, but realistically, as long as Partner is there, it doesn't matter. He is my choice now.'
You need to work on a way of devaluing her role in your life. It will be hard work (and I'd strongly recommend you book yourself in for some therapy after the wedding is all done), but you need to stop yourself being the frightened child in your aggressive mother's presence. You need to work on feeling like the valuable adult that you are.
Trust me, you really are valuable, able, loving and brilliant. Don't let anyone, even your mother or your father, tell you otherwise.