I also find it hard to say "no" to others. My therapist taught me about boundaries and I worked on strengthening my boundaries.

Also, my therapist succinctly told me that the word "No" is a complete sentence. I'd never really thought about it, but simply saying "No" in a polite yet assertive way is powerful. No need to explain or feel bad.

Practice it and it gets easier. I like, "No, I'm unable to _____." And leave it like that. It gets easier over time and it feels good to say "Yes" when that's what you want. Saying "No" when you want to decline is empowering and feels authentic.