Being a 'good mother' is a relative term, not a clear definition.
As my mother said, you try to be the best mother you can be under the circumstances. It seems to be clear that that is exactly what the OP was, the best mother she could be under the very difficult circumstances she was faced with.
No parent is perfect, we all make mistakes, our children vary in character and personality. About a month ago, DD was chatting to me and suddenly, quite dispassionately told me how I had failed her as a mother when she was in her teens. She then resumed our previous conversation.
It was so sudden, unexpected and odd, I was gobsmacked and didn't even mention it to DH for several days. He immediately pointed out several inaccuracies in what she had said. This corroborated errors that had struck me at the time. I also discussed it with DS who is very close to his sister. His immediate reaction was also to point out other errors in her memory, and like DH quoted chapter and verse to support his comments.
DD and I continue our close and affectionate relationship, but it is odd to think that one member of the family's memory of a certain period of her life runs counter to everybody elses. She does have a very bad memory and has said she can remember very little of her childhood. I can only assume she has memories of how she felt as a teenager and came to the conclusion, in the absence of memories, that she felt like that because of my 'neglect' and because she cannot remember me doing various things like helping her with homework, which I did, she has decided I didn't.
There is not much I can do to change her view of her teenage years, so I must live with it. The OP is in a similar, but worse position and I think, sadly, she just has to accept that that is how her son perceives his childhood and there is little she can do change it. However that does not mean his memories or view are right and none of us can go back in time to prove our point.