I'm probably totally projecting my DM onto you OP (poor you!!) but in my case, she's 'nice' when I'm upset because - this sounds dramatic but I believe it's true - she wants to feed off my sadness, it somehow pleases her.
Took me decades to realise that. Then I looked back and saw all the times she deliberately made me cry, then looked satisfied and walked away. And how she'd try to break me down when I was feeling happy. I learned never to mention my hopes/plans to her. Even earlier than that, I learned never to turn to her for help.
Quite honestly it's downright nasty that your DM hasn't bothered visiting you in four years (assuming she's capable of the journey).
An older ex-colleague of mine once said, when I cried at work yet again after seeing DM, "you know Postino, some people just aren't very nice". It was like a lightbulb going on.
I only know what you've written here, and like I say I'm projecting a bit, but I don't think you're overreacting or failing to let go of the past or anything like that. I think rather than you trying to change and feel differently, you could harness your perfectly appropriate anger and use that energy to build up your inner strength and resilience.
If you relate to any of this, there's loads of help out there honestly, amazing books etc.
Basically imo, it's not you it's her!