ah Puppy yes I know you've hit the nail on the head (as ever!). That's a really good question.
I am so used to being compliant, easygoing, "it doesn't matter, I'll take up the slack/annoyance/whatever" because if I didn't, people would reject me or get angry with me and I've never done conflict well (well, except for now, when I'm caring less about what people think of me or if they walk away).
Telling someone something that could make them feel less than happy (eg me having needs meant that they would have to accommodate them, which would be an inconvenience) would make me feel guilty for making them sad/inconvenienced.
So, yep, feeling guilty for saying no. Well, I did. I now just mostly feel confused about how to say no. I'm sure it's like a pendulum, swinging from too compliant to too (ahem) assertive, and will eventually settle. It's just that this new wave of angry assertiveness has come as a complete surprise!
Might get that book though, it sounds good.