If I'm not with her, I'm against her

[Wutnow32]

Dealing with my narcissistic mother today after she sent me a scathing email about choosing a step relative over her in celebrating important life events. That aside, I spoke to my wore about it, she was very sympathetic and felt bad for me. Later after I got home from work, the intensity of my emotions had subsided and I decided to just 'let it go' for now. Wife lays into me that I should be sticking up for myself now more than ever against my mother. That I shouldn't let it go, etc etc. Getting very animated while talking.
I try to explain that I'm not letting anything go that I just want to forget about it for now. She starts to say how's he's trying to be supportive and wanted to talk with me about it when I got home from work and now I'm just blowing her off and forgetting about it. That it makes her feel like I'm not on her side and an a outsider.
My therapist and I discussed this topic earlier this morning and she says that BPD individuals get a false sense of self when they are 'pushing back' against something or someone. When we were in solidarity about being against my mom, she was fine. But now that I have decided to just let things sit for a while, I suppose I've removed whatever sense of 'self' or 'power' she may have felt and now she's lashing out at me!
I'm not taking anyone's side but my own! I feel so trapped between my narcissistic  mom and BPD wife, I feel like I could scream! She ended up having a hissy fit and locking herself in the bedroom to 'nap'. I don't deserve this crap and I'm tired of it! How do I get her to understand that I'm on my own side and that I will fight my battles my own way, and not to take it personal if I don't go both barrels blazing?