How can I get my mother to stop bringing up the past.

Lavender,I do understand your frustrations with your Mother,I don't think that you couldn't Change your Mother's personality and her bad behaviors .....
But, look brighter side of her. She still wants to associate with friends!! After assisted her bathe and dress up, just simply say "Mom you looks wonderful"

Hi Lavender123, with my Mom when she gets difficult I tend to turn to humor. She thank goodness , still has a pretty good sense of humor. for instance when she starts talking about how no one is going to want to go to her funeral I tell her 'sure they will because I am going to dress you up like Wonder Woman in your casket and everyone will want to see that.' she just giggles and the subject usually gets dropped. But I do not think she has classic dementia yet. So that may not work with you.
Does your mother enjoy upsetting you? It sounds like she may be getting some sort of payback by riling you up. Maybe the best way to deal with her is to just not allow yourself to get upset. I know -easier said than done- but you can train your brain to remain calm and you can choose to be happy though being berated. I say this from experience. The more you breath through the anger and let it go the easier it gets , too. Did you know just by forcing a smile your brain feels better automatically. Maybe if you force enough smiles they will become real. And if your Mom is not getting the satisfaction of riling you up she may stop doing it -or not do it as much.
My suggestion is next time she says something about a past criticism I would just pat her on the hand and say"oh we'll, I tried my best!" and proceed to go about your business. Don't give her the power to ruin your good day! You can still have a good day -even if she has decided she wants to be miserable. Let her be miserable alone. Or just tell her if she doesn't cut it out next time you do her hair you are gonna spray paint it orange and blue and give her a mohawk! Heck-if she keeps complaining maybe you really should! -just kidding! Hope this helps. Good luck!!

Lavender, the past is where they live.... it's not going to change, and probably get more tiresome.... I know you are having a rough time, not getting any breaks... but hopefully you can redirect her to a nuetral subject.... maybe not.... leaving the room is a great idea.... it won't do any good to ask her not to bring it up... she'll forget you asked..... just keep coming on here and telling us how you feel and know you are not alone..... hugs to you...