Mental health vs Abusive

If your SO has mental health issues/a brain injury/etc, how do you know where the line is drawn between being an abusive asshole and behavior caused by mental health issues?

I'm having a really hard time with my SO (disabled vet). I'm not sure if I'm going to stay with him for the kids' sake and his or leave for mine and the kids' sake. He won't go to therapy and claims he thinks everything is fine in our relationship and I'm overreacting.

Any advice would be greatly appreciated.