'But OP can choose to think her mother abandoned her to an awful parent, so she could develop her business child free,
or
she can choose to think her mother tried hard to do the right thing but her plan didn't work and she was prevented from keeping her children by her ex.'
I don't think anyone's suggesting she think the first of these - not even OP herself. I think it's OK, and important for the OP (who is, I repeat, the one we are dealing with here, so the one on whom we should be focused, surely?), to say that yes, her mother did fail her, somewhere along the line. This is in no way to say that this failing is damnable, or unforgivable, or indeed not on some level understandable - but these are things the OP can come to later, once she has been permitted to acknowledge that the failing happened, that it hurt her and damaged her. I think if OP is pushed prematurely to a position of forgiveness because it is more comfortable for us that way - for any one of myriad reasons - she is not going to find peace, because while her mother's victimisation will have been acknowledged, her own will remain unheard.
To reiterate, as i don't want to be cast as not understanding abuse or sympathising with victims: The mother may well have felt overwhelming fear that seemed to leave her no choice in the matter. She may well have suffered a great deal over the years. But however powerless she was, her child, the OP, was more powerless, had less agency, less say, and was left living with an abusive man as well as suffering the wound of abandonment (I mean this in its sense as to how the OP would have subjectively experienced it) by her mother. If the mother had come to us, we could have reassured her that she went through a terrifying and impossible situation and urged her gently to reach out to her daughter now. But it's the OP who is here, and it's our 'job' (if we choose to engage in this thread) to hear her and not make it our first business to vindicate her mother.
(Sorry for so much third person, OP)