I can relate to what you say, OP. So here are some rather scattered and piecemeal thoughts.
My mother was violently abusive growing up, hated physical contact and was extremely intrusive sexually as well as being verbally critical. She would do terrible things when she got angry, which I won't put down here because of triggers etc.
I have a sister, though, with whom she is all sweetness and light.
One of the things that helped me was realising that she also had a dreadful upbringing herself and was to some extent repeating a pattern of scapegoating one sibling and treating the other as a golden child. I think reading up on this really helped me to understand how it wasn't actually about me, but about preserving a kind of dynamic in the family that was wrong and unhealthy, but about which I could do nothing. Now the key word there is the word 'dynamic' - your father's death will have changed everything for her, so suddenly you are moved from being the scapegoat to a very different, 'insider' position. This explains the sudden switch in behaviour. If you confront it, you will be told that it never existed because the most important thing for mothers who are abusive in this way is to maintain the fiction that everything is OK, and that any problems were external to them. (Control is a huge issue in these situations, and accepting blame equates somehow in their minds to a loss of it).
This is absolutely NOT to say that the feelings she's expressing for you aren't genuine. I think they very probably are and I also believe in my own case and perhaps in yours too that very, very deep down, my mother knows and regrets what she did. So the first thing is really: accept that things have changed, and that it's not your perceptions going wonky! And have a look at scapegoating as a concept, because it might be helpful.
Secondly, I think humanising the person helps. Illness, particularly depression, does affect behaviour. My mother was definitely mentally ill, and was also very socially isolated. The worst extremes of her behaviour happened when she completely lost control. Of course, she was deeply 'wrong' to act this way, but she was under a lot of pressure. Realising that mothers aren't always perfect is actually quite empowering.
Thirdly, as an extension of that, I have realised that it's easy to look at other families and think they are perfect and that we have 'missed out'. But that way of thinking only leads to endless and needless regrets. We have what we have, and we can only go forward and build a better life around ourselves.
However, as my mother has become more vulnerable, I took a very conscious and deliberate decision to forgive her everything that she had done, and to accept what she did have to offer as an older person. I also took a decision to contribute to the relationship without regard to what I 'got back' even though it is still very one-sided on my part. Now this might sound very altruistic and selfless, but it's not. It's actually the only thing I've worked that gives me peace and lets me get rid of the all the anger I felt (and my word, the anger was like a self-destructive hurricane blowing inside me at times - it nearly destroyed me when I was in my late teens). I feel like I love my mother very deeply now, but that I am also independent of what she says and thinks. If she criticises me and it's unjust, I just push it away. If she hugs me, I can just accept that without feeling regrets. I couldn't do that before - it would start off a chain reaction that would total me.
I think one of the most disempowering roles we accept is that of someone's victim. It means they define us. I'm not saying we should deny the abuse that happens, more that we shouldn't let anger become the thing that rules our relationships. That doesn't mean that feeling anger is 'wrong', just that we can sometimes move beyond it with time.
However, getting to this point took me 20 years so the final thing I wanted to say is that it's really, really OK to feel uncomfortable with this sudden switch and the feelings it's bringing up. It will take a long time to work it all through, for you will have got used to a status quo in which you had a very different emotional status, so this change is bound to be bewildering and a little frightening.
I also wanted to say that I think our relationship with loved ones and particularly mothers kind of bleeds into other relationships too. I started to practice (in quite a deliberate way, like you practice an instrument) feeling compassion in situations where it didn't really matter. Like when I got annoyed in traffic because someone was being selfish on the road, and I would just take a moment and try to imagine that the other person maybe had a terrible day, or just lost someone, and to be nice and wave or smile at them rather than getting irate. I worked up from these insignificant things to my mother. I didn't really intend to do this as a project, it just happened by accident, over a long time and it was only once I had got to the point of feeling genuinely sorry for my mother instead of angry that I realised what had happened.
Finally, this is what worked for me. But we are all different and your truth will be different from mine. So please just push away anything that doesn't feel 'right' for you. 