Boundary Training
Setting boundaries is an important life skill, so of course, they never taught me that in school.
I find it's easier to build your boundary-setting muscle by reading other people's horror stories; it's easier to spot the problem when you aren't in the middle of it.
Good places for me to read other people's problems and learn some handy boundary setting phrases are:
Captain Awkward for my personal life
Ask A Manager for my work life