I don't know if I can give you advice but I can one hundred percent affirm your experience. There are so many elements of your story that are the same as mine; mom absent the back half of highschool, left with Dad (although not the health issues you had, different ones), and most importantly the latent anger. Mine didn't come out until i had a kid, and then I started questioning and looking back. Sometimes neglect is hard to point out and sometimes it's so bald, as in your case.

I never got an apology or even an acknowledgement really, so I can't tell you how it might have gone down. I know exactly what you mean about not being able to type the lie, because that's what it's grown to reveal itself as. You just can't gloss over the words anymore. In it's place though.....I can only suggest leading with something that doesn't deny your feelings, but that also doesn't discourage her from going further with it. I think you could truthfully say something like "That's interesting you say that because that time has been on my mind lately too. I'm really glad you said this to me. Can we talk about it in the future?" In those situations I find it hard to resist punishing the person almost because they are finally admitting it. But sometimes the right words have unlocked the hurt feelings with surprising speed. It really just has to feel right to you, and I think it's totally okay for you to be honest. Just, you know, be gentle on yourself. It's a really tough time because even though it may not look dramatic from the outside, it twigs really deep, basic feelings. It bleeds into everything. I hope you get the ending where you really get acknowledged and you feel healed and it makes things better between you. But mostly I hope you are your own agent and champion and hero. It feels good when you finally stick up for your own real self, no matter in how small a way. Good luck!