Incorporating backstories into your fiction is an art. There's no real right or wrong way of doing it. But there are some best practices. But I'm not here to talk about those techniques.
I'm one of those writers who try to work everything out in my head, but sometimes there's just too much backstories to work through and keep straight. I find myself having to put my thoughts on paper (or in a computer file) to sort through all the stuff: the backgrounds, the interconnections, the relationships, the character sketches, etc. I also use a tool called Personal Brain to string everything together in kind of an organic way. It's a pretty good tool to lay out all these interconnected thoughts -- you can jump and set up links between thoughts.
If you find yourself struggling with lots of backstories in your head, try to set the actual manuscript aside and start on a new file/page, and start typing about the backstories, things about your characters, etc. etc. Free style. Stream of consciousness. Whatever that comes to your mind.
But just get it all out. And it will be a work in progress. As you continue with your main ms., you can refer to that and change things. But at least you'll have someone tangible, and not just a tangle in your mind.