Well coming from the parenting after a certain point you can’t really be close to someone no matter how long you’ve been together, I get where you’re coming from but after a certain age the child will feel detached and not wanted so he will stop approaching and since the mother doesn’t really care she will too and even if she starts caring it will feel awkward to try and get involved into his life and even guilty at some points
I think the post partum depression, added with how she couldn’t return to the life she had just piled up and her resentment towards her son grew.
It might be because I'm not a mother myself, but ...
As I've understood she got married at 16, gave birth at 17 and was 22 when the kid was 5. I think even back then she was quite young to marry & have a kid. So I understand that at the beginning she had difficulties to cope with her situation and even hated her child.
But wouldn't one get the situation at least after 10 years or so? When the child doesn't seem to need you and if you don't have the courage to suddenly open up (I'd totally understand that), wouldn't you at least try to get to know him from the shadows, like what lessons he has or if he has any friends, etc?
Was she for 18 years after those superficial parties? When the ladies are young it could have been quite fun, but after some time almost every lady turns into a snake, don't they?
It's quite a pity, but since she had the education of a successor (which is rare for a lady), she could have started a business, could have helped out her husband with his job, become an investor, a scholar, a writer or sth. Through that she could have neglected her son...
Well, that's my opinion though, and I'm not sure what she is allowed to do as a lady.