Silverfish I really feel for you, I had the reverse situation.
My mother was very, very controlling until she was in her 80s and she had a very minor stroke. It changed her personality completely and she actually became a much nicer person and, oddly enough, developed a keen sense of humour which she had never had previously. My DD and DS couldn't believe that she suddenly 'got' jokes and even made funny remarks! She became far more amenable and less accusatory and just generally more relaxing to be with than ever before.
Like your mum, mine had always had loads of friends, too, and played Bridge regularly and belonged to an art group. When she died, so many people commented on her being 'quite a character' and that she had 'very strong opinions'. People like that are often loved by friends, as they are outside the family situation, but 'characters' are very hard to live with!
As others have said on here, do you think there could be a health issue? Don't want to be alarmist, but possibly even a mini-stroke? Would she be open to the suggestion that she should have a health MoT?
It's so difficult, isn't it, especially when they have nice characteristics as well. If they were truly awful all the time it would be so much easier to do something, such as make a stand.
I hope you mangage to find a way to cope with all this 