Mum pretty much whole-heartedly has hated everyone I've ever gone out with it, so it's just intensified hate with multiple partners
No, honestly, I get where you are coming from. It is hard to face disappointing your parents, especially if you are economically and emotionally entangled with them and rely on them for practical support, like you do.
For me, the biggest helpful realization has been that I don't owe my parents anything. The decision to bring another human being into this world and to raise it is a free, voluntary decision, whereas a baby can hardly decide if and to whom to be born. Parents owe their children, not the other way around. It is not your responsibility to make your mother happy, if you could even theoretically do that for another person in the first place.
I have the same situation in that my mother lives very close by and is very up-to-date as to the goings on in my life. And since I've never been able to lie to her very well if at all, she immediately caught up on the whole poly thing. She has reacted a bit differently to each of my relationships;
1) Sweetheart; "What an irresponsible twat, to start sweet-talking another woman while his wife is left to take care of the kids"
2) The Sweets; "You are being used, moron, to pepper up somebody else's worn and tired marriage, and will be discarded in all due course"
3) VanillaIce; "I don't want you to be seen anywhere with HER. I mean, why can't you just get healthy already? Get yourself a nice man and two kiddos and make me a Grandma"