"Im sorry that you feel that way" is a good thing to keep in mind.
Also,
"I can only control how I react and how I live my life. Other peoples choices about how to feel and react are theirs"
"I am making boundaries for my own health and self preservation"
"I cannot be a helpful person to anyone if I am not taking care of myself."
You dont have to say these things to anyone but yourself.
No one NEEDS to know or understand why you are making youor choices. They are yours.