Learning to say "no" is a hard thing. Many of us aren't good at it. But then we become so overwhelmed, we practically say 'no' to everything because we haven't learned to say 'no' to anything.
Recently I heard the quote below and it helped me think about saying "yes" differently. Maybe it will help you.
"Whenever you say 'yes' to anything, there is less of you for something else. Make sure your 'yes' is worth the less." – Louie Giglio