It wasn’t ‘all about your happiness’. It really wasn’t. You were in a bad relationship and that would have been impacting on your DS, kids aren’t stupid, they know.
If you have 50/50 care of DS then you need for his Dad to do 50/50 of the parenting. You need to tell DS that homework & reading needs to be done at Dad’s house too and you need to tell your ExH that he’s letting his DS down by not parenting him & not doing homework.
If the shared care is ExH having him every weekend, then that needs to change.
Try to do little things to make life a bit more ‘fun’ and less of a chore, but you don’t need to go overboard. Kids aren’t stupid. They know who is looking after them and they know who cares. They might have more ‘fun’ with one parent, but they know which one is always there for them, looking after them, making sure they have all the stuff they need etc.
It’s hard to know what to suggest as ‘more fun’ without knowing you or DS, but things like Friday nights are Movie & Popcorn nights, or in the summer Park & Picnic nights. Saturdays are pack a picnic & go for a bike ride, or breakfast in bed with cartoons. Or swimming or whatever. Kids even enjoy just doing regular stuff if they’re doing it with you. Teach him to cook, let him plan the weeks meals & go shopping together but with him ‘in charge’. Decorate a room. It was those things I loved & remember from my childhood, far less so the days out etc.
You did the right thing, TRY to stop feeling guilty. DS is better off with things like this. Truly 💐