Of course she had a rotten childhood, people rarely get to be that messed up without help.
The thing is, she is a damaged person, who has an established track-record of inflicting damage - mainly on you.
I think, like a lot of posters here, that counselling would be very good. I can see why you don't want to spend money talking about her - it would kind of crystallise a sense that she continues to have an effect on your life, when I suspect you want very much to prove to yourself that you have survived her, and managed to live beyond her effects.
But she clearly is still affecting you. And I think I can see why you wonder about whether you are doing the right thing about contact (which I think - for what it's worth - you are), part of you really would like the dream that it could be made alright, and you could have, in some form, what you were denied as a child.
Counselling can help deal with that. But I wonder if you also feel wary of counselling because it might bring up many feelings and truths that you (feel) you have to suppress in order to be able to get on and live your life? You are probably right about that. I wonder if counselling might not be a short-lived processs, and be quite intense.
I do wonder if it might still be worth it though. You so clearly realise that you are right to keep her at a distance, and act on that, yet she also clearly has the power to frighten you, and also you doubt the wisdom of your decision (though, thankfully, don't act on that doubt).
I had (some) counselling, rather late in life, and discussed my relationship with my mother. I had resisted it for years, and really empathise with what you wrote about not wanting to pay to discuss your mother! I found the experience of counselling helpful but, yes, a bit shocking. I wish I'd done it sooner. I didn't cut off contact, and I rather regret it now, to be honest.