Oh how so much of this resonates. In this situation I am the daughter, and growing up feeling like you don't quite belong anywhere is really damaging. Feeling that love is conditional, and that you are noone's priority.
However, this really doesn't sound like any of this was deliberate - it's unfortunate, children can become collateral damage when big things happen, such as marriage break downs, divorce, death, new relationships, new homes, etc.
I went through many of the things the OP's daughter talks about, BUT, I know while my mum and stepdad did get things wrong, it was never intentionally to cause pain or because they didn't care enough. My dad, however, has repeatedly chosen to prioritise himself and various partners over his child again and again - that is a very different situation.
Like the OP's daughter, I had therapy to unpick up some messy thoughts and feelings. Talking to my mum, she was quiet, understanding, keen to listen, and when she'd taken it all in, apologised for where she had got it wrong, believed my experiences and empathised with how hard it must have been for the child me, and told me she wished she'd done better but she had always tried to do the right thing with the right intention, though she could see where it hadn't looked or felt like that for me. That conversation healed me.
My dad and I have continued to have a difficult relationship and when I recently told him how his actions, or lack of, made me feel (then and now), and the reaction was the exact opposite. His new wife sent an email full of rage, accusing me of wanting to cause hurt, telling me what a disappointment and embarrassment I am and how this hysterical, ridiculous drama must stop and to stay away. He has remained silent, and our relationship is over.
OP, your daughter had a painful time as a child. She's still carrying it now, and she's looking to you to help her. This shows her love for you - she's asking you to listen, to hear her and to help her heal. If you meet her with anger or defensive explanation, your relationship won't recover. If, however, you really listen, empathise (just saying 'sweetheart, that sounds really hard' or 'I can see why you felt so alone' will make the biggest of differences) and apologise (even if it's a 'I am so sorry you felt like this for so long' or 'I am so sorry that I didn't see this at the time', this is absolutely repairable.
Listen properly and try to understand - this isn't about her blaming you, this is about her trying to make sense of a deep-rooted unhappiness, and needing to hear you say that you wish you'd done differently. Don't let it ruin what you have now.