Thank you for the replies. There is a lot to think about but it's helpful to get other people's perspectives.
In answer to some of the questions/points, I don't think it's necessarily that L doesn't value me or consider me important. It's more that she seems oblivious to other people's unhappiness or distress until it's spelled out in capital letters to her. And I do think it was a genuine accident. I think part of the reason S has stepped back is that he can be rather down on himself and feels he isn't 'good enough' to be a father as he isn't working at the moment and has had some depression. He is lovely though, and a truly well-intentioned human being.
I think we do need to talk a lot more though over the next few months and I need to try and really say what I want/need rather than what I can tolerate.