yes is no
I know you think you have a hard time saying no to things, but that’s not true. Every time you say YES to something - a request of your time, your energy, your money, your power - you ARE saying no to something else (usually yourself in some way).
They are two sides of the same coin and you can’t have one without another.
Experiment: The next time a request is made of you, say whatever you have to say to buy yourself a bit of time before answering. “Let me check my calendar”, “I’m not sure I’ll let you know later today”, “I’ll call you back in ten minutes!”. Then, during that pause, take ten deep breaths and answer the question, “if I say yes to this request, what am I saying NO to?”
Let that list guide you.