Nitalk, welcome to TP. I hope you find good advice and support here; there is plenty of both.
I'm so sorry to hear about your situation with your mother. I found it reassuring to learn that I am not the only person here, who has a mother who is/was nasty and unpleasant in the past. Of course dementia is difficult enough to deal with, without the baggage from the pre-dementia relationship making it worse.
I don't care what your mother did or did not do for you in the past, you have a right to your own life and your own happiness. My unsolicited advice for you is absolutely to cut down on the visits for now, both the frequency and duration. At the first sign of nasty behaviour, I would be out of there like a shot. I always remember something my aunt told me, about daily visits to her own mother (not dementia but just unpleasant) in the CH: that when her mother would turn nasty, she would calmly say something like, "I can see you're not up for visitors just now. I'll come back when you are feeling better," and then my aunt would leave, and not visit again that day or the next. It didn't take long before my grandmother began to behave herself!
While I'm not suggesting that behavioural conditioning will be effective with someone with dementia, it does work both ways: if you can condition yourself to remove yourself when things get ugly, then you have still made the effort to visit but you are sparing yourself the worst of the comments. I find that those comments, as trifling as they may seem in the grand scale of things, are really, really challenging to deal with (both in the immediate moment, and afterwards). I frankly prefer to avoid them, and who wouldn't?
I suggest you simply leave when things turn ugly, and even if they do not, keep your visits shorter. Other strategies include not visiting alone (my mother is much nicer if my husband is there, as she likes him), having something ready for distraction, such as suggesting a cup of tea or a change of scenery, and having an exit strategy ready, if that helps you. I also make a lot of trips to the toilet because just absenting myself for a minute or two can help me to calm down, and sometimes I don't have to go, but just washing my hands or texting a friend or surfing the Internet for 2 minutes is a good distraction. Usually when I return, my mother has lost the thread of the conversation. Plus, she can't remember that I just went to the toilet, so I don't get snarky comments on that, thank goodness!
So I would say that for now, don't visit twice a day, and don't even visit every day. You need to have days that are free of visiting. I hope you will at least consider this option.
Guilt can be paralysing and terrible. Many on here speak of the "guilt monster" and we all struggle with it, to varying degrees, at different times. It's easy to say, but challenging to do: don't let the guilt consume you. You are not a bad person. This is not your fault. Dementia is the enemy. This is the fault of the dementia, not you.
Best wishes to you, nitalk.