My mother gets very mad at me when I mention how close our family use to be and how things have changed.

When someone has dementia, I'd try to steer clear of topics that upset them. Try to agree with whatever she says, even if you really don't. If you want to discuss your actual feelings, do it when she's not around or when you are with other family members. When the brain has been damaged, she's just not able to properly process information or filter her thoughts and behavior. If she seems upset with something you say, quickly change the subject to something that she finds pleasant. You can find some tips on this on You tube with Teepa Snow on dementia care.

It sounds as though you may be upset yourself about how your family has changed and is no longer close. Mixing your own feelings up with your mother's difficult reaction is going to make it worse for both of you. Is there someone else that you can talk to about your own regrets?

Best thing to do is not mention things that upset her.
Do not know what the rest of your post was but I found that trying to discuss something or argue about something with a person that has dementia it does one thing...frustrates both people.
If you come across a topic that upsets her, change the subject. If she still goes on let her talk on do not respond with a comment that could be taken either way. Just ask her if she wants some tea. If she continues ask if she wants sugar in the tea. You get the idea.
It is difficult not to get caught up in the talk but for your own sanity stay neutral as possible.
Ear Buds might help, listen to some music, keep it down so you can make out some of the talk but it will take your mind off the "discussion"