I think that "Saying No" is a tough one.
It's finding that balance between do what is right/best in a situation when we don't want to do ??? and saying "No" when we are excising our "Rights" to use our time, energy and skills for taking care of ourselves, doing what's right for ourselves.
In addition to "A Women in Your Own Right" "When I Say NO I Feel Guilty" by Manuel J Smith is a good place to start.
I took both these books to the charity shop last week as I no longer teach Assertiveness, I hope someone benefits from them.
Undoing years of social convention, family norms and or own view of ourselves is no easy task.
Saying 'No' is a skill that was not often taught to (mainly) women of our generation so we find it harder to learn but we can work at it.
It's not easy to set "Boundaries" and to maintain them, saying 'No' is all part of setting boundaries and taking care of ourselves.
Realising we have the "Right" to take care of ourselves is a Big mind shift for some people and it takes time as well as practice.
So often the message we were given was that we "Should" take care of others. A better/healthier message is to find the balance of taking care or ourselves and of others too and how to juggle those Wants and Needs.
I do think most of us have a good radar for when our kindness is being misused, it's a problem at that point if saying No is still difficult. Giving and caring, showing kindness can bring many rewards to us but being taken for a mug does no-one any good.
It is OK to stand up for ourselves and say "NO" and for us to be the one to choose when we use our "No".
For those struggling with saying 'No" think of it as learning a new skill. At some point we had to learn to write, to drive, to cook etc and we needed to read, watch, practice until that ??? skill was easy for us. Assertiveness/saying No is no :-) different.
Go online, read the "Bill of Human Rights" look up articles, borrow/buy books then find someone you feel safe with and practice, practice, practice.
If anyone feels like a doormat and is beating themselves up about it please don't do that to yourself. If you've never learned to fly a plane you wouldn't beat yourself up for not being able to fly a plane. It's the same thing.
I bless my Mum for teaching me early to say 'No' in small ways and going through my working life I've really had to grow the skill.
It is never too late to learn a new skill.
PS I can crash a simulator but I can't fly a plane :-) OH can fly a plane but can't sew a seam, he can manage a button but that's as far as his sewing skills go.