I've worked on assertiveness in my own therapy. Much introspection is necessary because it's truly helpful to understand your needs in a sense of knowing where you draw the line about what you will and will not accept as far as how you are treated. Boundaries are really something internal. I think Rose mentioned consequences as far as what others know.

It's ok to say, listen, these last minute cancellations and noshows are really testing my limits and I'm questioning how valued you find our friendship. I might find myself making other plans from here on out. I don't like the way this leaves me feeling, etc. (assertion)