Sorry you are upset op. A mother-daughter relationship is so fundamental that it is very distressing when it goes awry.
You describe your mother as being "largely absent emotionally" but then you say, "She's always been too involved with her kids personal lives" so maybe your mum is getting in too deep now, in the mistaken belief that she is making up for previous mistakes? And doing it badly?
I think it's hard for strangers on the internet to comment fairly because you understand the ins and outs of your relationship with your mum much better than we do. It goes without saying that if you feel the need to go nc to protect your mh then that is your right and you absolutely must do that.
But to comment as the mother of two adult dds, with a different perspective, I think a lot of this is about intention and changing parenting styles.
Intention is important because it gets to the nub of why your mum isn't supportive about your reason for getting divorced. You say she is looking at you with hate which must feel horrible. And you are hurt that her loyalties seem to be with your soon-to-be-ex and not with you at a time when you most need support. Do you know what is going through her head? Do you know what the intention is behind those words?
If she truly hates you and is saying this out of jealousy because she didn't have the balls to leave her DH, then you would be right in putting a distance between you. But if she is genuinely concerned and worried say about your financial security, or is anxious it will be very hard for you parenting without your ex living in the same house, or she sincerely thinks you are making a big mistake, because she gets on well with your dh, and only sees the nice "public" side of him, then she is guilty of foolishness but not deliberate hate ifyswim.
One of my sister's has an over-developed sense of fairness and when I severed a friendship once with someone she knew, she wanted to be scrupulously fair to both of us, and I was livid with her for not having my back, but she said it was because she saw fault on both sides and wanted to behave the same with both of us.
It didn't mean she didn't love me, but I found it very upsetting at the time.
Nowadays, looking back, I can't really blame her, if she did genuinely think I was at fault too, because she had an independent relationship with this person. If your mum has developed a friendship over the years with your ex, is she wrong to express sadness about it ending? I can't answer that because it depends again on the intention behind her words and how she phrased it. If she was just thinking selfishly about herself, and not the more serious implications for you, obviously that would be very wrong of her.
As for changing parenting styles, nowadays parents are seen as friends who should always have your back no matter what you do. But back in the day, parenting was more objective and unbiased inasmuch as the emphasis was on producing a good citizen who was polite, productive, hard-working, etc, and the emphasis wasn't so much on the mh or happiness of the child concerned. So if your mum is old, maybe she is applying old fashioned standards to your split, and thinking about the implications for your ex and your dc, and wider friends and family, rather than putting you at the forefront, which is obviously very hurtful because you have obviously considered all of those things. But again, the intention may not be hateful.
I don't know if any of this is helpful op. A lot of it could be wildly off track so feel free to ignore. But I very much doubt your mum hates you, even though it feels like that. 🌷. Even so, it's your right to go nc if the distress she is causing you is unbearable. But I think it's worth trying to have a calm chat to her about your feelings before you do.