My 86 year-old dad tells the same d*mn stories all the effing time. He has no idea how offensive it is. He listens to nobody else. He doesn't care about anybody else. Before you say it is his age, he has always been self-centered. So it is hard to GAF now. I feel so sorry for my mom who has to live with him. I don't want to go over or take him to the doctor or for haircuts anymore.
If it's just the tedium - and you're not having to pretend in public that you've never met him and have no idea who he is because you're so embarrassed by what he says - then I'd have to be harsh but fair and say come on, give your poor mother a break and take him off her hands for brief periods - it may be screamingly boring but you'll live.
If he's saying things that offend other people (as well as making you cringe), though, then you and your mother will need a different strategy - like enlisting his old friends' or workmates' families, neighbours or paid helpers to take him places and laugh at his jokes. But don't run away - your poor mother!
1. You can't reason with Dementia
2. Dementia makes regular jerks bigger jerks
3. When there's a parent/adult child history of relationship problems, there has got to be a caregiving plan in place that doesn't force everyone to be too close too long.
You say your mother has dementia, but is she still living at the house with dad? Do they get any in-home care or assistance? How many years has your mom had a dementia diagnosis?
A little more information can help us help you.
Speaking generically... There comes a time in every aged person's life where changes are going to have to happen to keep them safe and well. Not a lot of people in that category right now ever planned on it, don't have their affairs setup, and have left it 100% up to their kids to figure out. And will fight it every step of the way.
My first suggestion is to contact your area agency on aging and ask for help. Ask for an in-home assessment to understand what options exist in your community to help your folks out.
See about getting mom & dad into some respite care for a few days or a week, so you can take a break for your own healing & rest. It is typical for elders to refuse to go along with any of this willingly because they can't understand anymore. But, if you are in the nuthouse, you'll be of no use to anybody.
We are hear to help you, listen to you vent, and to be supportive as much as possible.