Advice to be more assertive?

I've posted here before because my nonconfrontational ass fumbled a bag and here I am again, having fumbled the bag because I didn't ask for or demand what I wanted for my time.

I don't mind keeping customers company as long as I am being adequately compensated. Unfortunately, I am not good at asking for what I want and being assertive. I want to fix that!

Does anyone have any videos or articles that helped them reinforce financial boundaries? Or any motivational speeches about asking for what you want, when you want it? Or book recommendations?

I'm kicking myself so bad for allowing people to take advantage of my time and energy but I don't know how to actually make myself demand better treatment!