Hi, I'm so sorry for what you're going through, there's nothing more all-consuming than child stress.
My cousin was really difficult for a while. When I came over he'd be under the table kicking me, he was constantly stressing his mother.
I don't know how, but she kept engaging with him (and they saw a therapist, and there was an issue within the household) and he grew up to be the nicest kid. But I know that saying 'time' isn't the most helpful.
With ours, when we were having some difficulties we found that spending one-on-one time with each of them. Not doing anything, but just spending focused time with them. This of course requires outside support to keep the other one.
Good luck, and you're not alone.