I've also had a very difficult relation with my mother. As my father was always abroad for work, when little I only had her and I loved and trusted her completely. Only later did I find out how much she had betrayed my trust, and then I had to begin the long journey of going back to myself from where she had taken me to, which was a place of great pain. It is the same place where she lives herself, of course, but feeling tender for her for this only slowed down considerably my process. Since age 18 I've lived in another town, which helped. Now I even live at the opposite side of Europe. Lately, I've entered in great crisis after the birth of my son, because I resonated more deeply with the sufferance of my child-self.
I suggest you to put physical distance with your mother, of course your ghosts will come wherever you are, but at least you'll be able to face them from a point of better serenity. Forgiveness will come in time, but the phase of anger is very important. In my experience, things go in cycles, where some anger will be absorbed by forgiveness, allowing me to go deeper and uncover new anger again that will eventually take me to deeper forgiveness, and so on.
My mother is a bit like yours, apparently sweet and full of attention, in reality full of resentment and highly manipulative. Her double face has fooled me more than one time. I found myself again and again doubting of everything and feeling awfully guilty for ever thinking bad of her. How many times I wished I had a less cunning adversary! sometimes she would really drive me mad, also because there was just my truth against hers, and no witnesses (my father divorced at a certain point).
Her family (which might be also mine if it was not hers) thinks very bad of me. They all live in a different town, and had always the filter of her words about everything that ever regarded me. At some point, she even dared calling my dearest friends, with excuses, playing the victim and insinuating I was being bad with her because in reality I was angry with myself and so on. And my friends calling me and saying poor her, don't be so hard she's your mother after all...she did the same with my husband, she sent him a letter saying all these same usual things, that she's never done anything wrong to me and that I'm having problems all in my head. Whenever she saw me and him together, she just always tried to separate us, insinuating he was too good for me. (he IS good in fact, and never was fooled by her millantery)
Well, maybe your mother is not so extreme as mine, I hope. Anyway the pattern is that one: slander and manipulation, as you called it.
What I did, at a certain point while doing psychoanalysis, I began to fight her, and since then, I've always been fighting her. I also began at the same time my spiritual path, to which I'm very committed, I've used meditation, shamanic sessions, many various techniques of dissolving negative emotions, you name it I tried it. I've forgiven her at some levels, but she is still there, my babau. She's become more and more bitter at me as I fought her, of course, and any relation with her has rivealed impossible. I understand also based on my experience that to fight directly is not a good way to keep the relationship clean, but if I went back I would do the same, only more radically. Then, if there is a relation to be saved, it will survive, if not, let it be. There is no writing or visualizing that counts as to look inside the eyes of your mother and say: you don't do this to me. The people I know who didn't do this firmly enough, are not living fully their lives. It's as simple as that. I might have lost a mother, but I saved myself and I'm living a very prosperous life now.
Somehow these difficult mothers are mirroring your relation with yourself, where you contain both the roles of the mother and the child. They seem to have created them, but I believe that they are partially pre-existent, and you chose this mother for this incarnation to dissolve the issue by putting it clear and loud in front of your eyes. There is probably no limit to how you can grow as the mother to yourself, having a child for me now is a great teaching, because I can deal with the same issue inside and outside, but it is also sometimes very painful. Very often points of pain come out, that I cannot motivate, it is just all this sadness stored inside me, that comes out without a reason. Yesterday we went to the circus and at a certain point I just couldn't refrain from crying, for no apparent reason. My child is a brave and deep soul, he's risking something having me as a mother! but he seems to know very well what he's doing...
Recently, I have discovered a real pearl. It's a book by Vimala Rodgers: Your Handwriting Can Change Your Life. When I bought it I thought it was a silly book, but took it anyway because when little I was very fond of my handwriting. In reality this is the most amazing book I've found in years. It is simply magical. The thesis is that if you handwrite a certain way, every letter resonates with an area of yourself and produces deep effortless changes. I knew some alphabets, like rune, have magical properties, but that also roman alphabet can be magical, well, that was new. I began to exercise 3 days ago and I'm amazed by the changes.
The letter "c", the letter of divine trust, is also the one that regards the relation with your mother (isn't it surprising? divine trust is associated with the mother, how far this can be from my experience...). You should write the "c" without hooks, just simply the 2/3 of a circle. To avoid the hook seems simple but for me it isn't easy at all. Write pages of them every day and you'll see the difference. Of course, you might desire to write also other letters, which can help you develop the qualities that need to support your issue. The book is small and quite inexpensive, have a look:
good luck to you