How to cope with someone with mental health issues.

This happens from time to time. My grandson is so unpredictable, an innocent comment can result in a verbal assault, it’s almost predictable. I let it go as he always comes round but I live on a knife edge a lot of time. All other family have no time for him. When he’s ok there’s not a nicer person, but he turns on a sixpence and I get depressed myself. Very often I feel that’s it, but I do care for him.