Hi OP. Some similarities in your situation and mine. I have a very toxic mother, and sister. I won't go into details but I really emphasise with you. I went NC with my mother and sister 6 years ago. I can't begin to tell you how relieved I feel. It's like a rock being lifted off my back.
Maybe in future I will feel sad about it but, who knows? A friend said to me recently wow, you smile a lot. I actually hadn't realised that for years, I hadn't really smiled. I now focus on my lovely DDs who are in their early 20s, doing well and happy . My lovely OH who I've been with for 5 years. There is bliss in knowing that, he won't be meeting my mum - so she can't do as she did previously ie belittle me in front of partners and absolutely gloat when relationships went wrong. OH knows my story, doesn't judge and is really supportive. We are happy.
I'm convinced if I didn't go NC, I wouldn't feel as happy and peaceful as I do now. Everything feels ok now in all aspects of my life. I don't know your age but I'm in my 50s. & I should've gone NC so many years ago. I was always afraid to.
Don't leave it too long, will you? & ignore anyone here who calls you entitled, or conveniently ignores that yes you have been less-favoured and that's hurtful, or infers you should prostrate yourself in front of your mum, explaining the hurt etc. You know your mum will thrive on it. & nothing will change. Ill-behaved people know what they're doing, there is absolutely no purpose in thinking you telling them, will change anything.
& unfortunately there are people out there who will read a story like yours and stick their oar in to put you down. Compounding the shit others have been through makes them feel good
Have you been on the Stately Homes thread? You will get a lot of understanding over there. People who have gone through similar to you.
Concentrate on yourself and your DCs. That's where your real happiness is. Live your best life, as they say. & you don't need people around you who don't mean you well, in order to do that.