I really love this topic!
Indeed, it's so important to say NO in order to define boundaries and personal limits.
There is no such thing as the "perfect child" :)
For a long time, I also had trouble saying no to people because I was afraid I would upset them. Turns out that by not saying no, what they knew as "me" was a distorted image of reality.
So now I say no when it's needed.
Did you read the book "The 4 Agreements" by don Miguel Ruiz? One of the agreements is "take nothing personally" :)