I can totally relate to your post.
I haven't had contact with my mum for five years. Like you I was also physically abused but it happened when I was little. She would sometimes stand by and watch it happen but she was a victim of domestic violence before it started so I thought she was scared to get involved in case he turned on her and I forgave her. Over the years however I realise she is/was absolutely besotted with the guy and would do anything just to get any hint of attention from him.
I was assigned a mentor to help with my self-esteem and confidence when I was eight and it would take seven sessions before I spoke a word to her because I was so scared she would be the same as my mum. But over the five years of her taking me places and treating me like a human being and not a complete nuisance as my mum did I became very attached. She moved to Australia with her husband and daughter when I was thirteen and I was absolutely devastated. In my head she had become my mum.
We are still in in touch now but it is so difficult. I am still quite attached and she is still the closest thing I have to a mum in my life at present but she is detached. Like you I don't blame her and I know the problem lies with me because of the issues with my mum I want and expect too much.
Over the past few years though she has told me that she thinks of me as a part of her family. She visited back in 2013 and introduced me to her daughter as her sort of adoptive sister which was quite a shock as it was the first I'd heard of it. But she doesn't treat me as a part of her family, I am more of an outsider and it hurts.
I have tried therapy for my attachment issues but I have had little joy to be honest. I have spent the past two years ot so distancing myself from social media just so I don't stumble across her posts much.
I am not sure what else to do, either apart from cut her off completely but I have tried and tried and each time I fail. I don't think it will ever happen.
She has said she'd like me to visit her sometime in the future and I could stay with her family but I have said no as the attachment would tear its ugly head and I would only have a breakdown like I did back then from the non contact.
I long for her to be the mum I never had like she was when I was a child but I know that is never going to happen because she doesn't feel that way and it is affecting life as it has done for the past eighteen years or so.