I have to fix this

I have a painful thought at work. I am a supervisor so whenever somebody is unhappy I think I have to fix this.

C Staff saying things
T They are unhappy, I have to fix it (but I don’t know how)
F Worry
A Spend all my free time stewing over the details, think of things to do but never do them, buffer, doubt my ability to lead
R Gain weight, urges to run away, don’t enjoy my job

In this case, it is my friend who is doing contract work for us, she has said she won’t renew the contract because she doesn’t receive enough support and she doesn’t like the way some staff members act.

So I know that that is her model. I don’t think it’s something I have done. So the thing I need to ‘fix’ is the other supervisor’s model and how he has acted towards her.
I know we can’t change people and I have long ago made some kind of peace with his behavior. He is now my partner in the business.
But I also don’t want to keep losing staff members because of the way he behaves (throwing things, not greeting people in the morning, shouting at people occasionally)

I guess in a way she has just created a boundary. She is leaving us.

I’m just trying to figure out how to deal with this personally. And also for the good of our small business, where it is difficult to find someone to replace her.
Sorry this is not a very well thought out question but maybe you can help me figure out where my role in this is and how I should direct my thoughts and actions.