Have you looked at the skills & forms associated with the DBT section of interpersonal effectiveness? I have found it to be a great help in communicating "NO" & communicating needs.....though it still doesn't come easy for me but my challenges are a little more complex than "no".
What you did is an awesome start.....good for you.
Lol....lots of times I really don't feel like doing something but am on the fence about it because I enjoy doing those things......but I realize what a relief when the others are the ones who cancel.
Guess I don't want my NO answer to come too easy because I could get caught into my NO, not doing anything mode & that is hard to break for me too.
My daughter got used to my "no"answers growing up but I always qualified the "no" with a "come to me with a logical reason why I should say yes & I will reconsider" she learned to approach me logically rather than emotionally & usually convinced me that it was actually ok. It was an interesting & worthwhile exercise for both of us.