Start with your husband. Make it very clear to him that you will no longer tolerate his mother treating you like this and he can either support you who is his wife/mother of his child or go back to her. He can't be married to you but allow him mother to run all over you and him. If he refuses to pick you, then you have some serious decisions to make regarding your marriage
Yes, do this OP.
Then go NC with MiL. No discussion, no negotiation, no "trying to". You just do it. You don't need anyone else's permission.
When she reacts by sending her Flying Monkeys at you, don't respond. Block all of them too. MiL & her hit squad are not your responsibility. Remind DH that it's up to him to handle his mother, because you have spent year doing exactly what she asks, but she's still going out of her way to play headgames & make you unhappy.
Ultimately, DH need to step the fuck up, & you need to work on your boundaries.
Once we moved out I tried to set some boundaries with her as she expected me to go round to see her every day and I tried to stop doing that and she got my partner round to her house and slated me to him saying how much I hate her and so on, so I gave in and started going round again.
Then you didn't maintain a boundary. You gave MiL permission to trample it - you did not hold it. All you have done is teach her that the nastier she is, & the more she uses your own DH against you, the easier you will cave in to her.
I've tried to reduce the amount of contact I've had with her in the past and she's kicked off to my partner and she's always got her own way
Tell your partner, in simple, firm words, that you are now NC with his mother, won't be coerced into having a relationship with her, & no matter what she tells him or how she manipulates him, you will not be changing your mind, & he is to stop banging on about what his mother wants from you & start concentrating on supporting his own wife.
You are going to have to get a lot more hardheaded about this OP.
Let her kick off to DH - not your problem.
Stop giving her her own way, just because she involves your DH.
Nobody can force you to interact with this woman. But you are the only person who can ensure you no longer have any contact with her. If that upsets DH, tough tits. He should have considered your feelings & needs years ago.