I'm also interested in what you have done to demonstrate how sorry you are. I have had a difficult relationship with my mother for similar (but not as serious) reasons and one of the things I find so infuriating is her inability to properly apologise for how she was. She pretty much refuses to discuss it, makes veiled references to things, puts a spin on events to minimise them, and changes the subject if I try to tackle it head on, or turns on the waterworks and tells me I don't understand how hard things were for her. She once sent me a card out of the blue saying she hoped I'd had a happy childhood; anyone who knows their child had a happy childhood wouldn't need to do that. What would make a huge difference for me is a proper heartfelt apology about her behaviour and it's impact on me. I know I won't get one though, she just isn't capable of it, and that defines our relationship, I'm now nearly 50. I would imagine she feels she's apologised, whereas to me what I've received are excuses not apologies.
We do have a decent relationship and we speak regularly (we live far apart) but I control how much I let her into my life, which I know upsets and frustrates her particularly as she's getting older and wants support and to be closer. She's changed so much and is now very much a sweet old lady, but that doesn't change my childhood at all. I still find actually seeing her incredibly stressful even though on the surface our relationship seems ok and works fine over the phone.
But, this is how I need things to be to be happy and mentally healthy, and it's my absolute right to do so, the same as your daughter has the right to remain NC with you for the rest of your/her life if that's how she keeps happy and healthy. You do not get to be the injured party here because her NC upsets you.
It sounds like you are taking on board people's advice, which is great. I really really encourage you though to think about this from your daughters perspective, not yours. Your apology needs to be genuine, deep, and about her, not about you. All you can do then us leave the door open; everything else then is up to her and you need to respect that.