Be assertive! Don’t use up your life on things you don’t want to do!
Every time you say yes to something you are then going to spend this bit of time NOT doing everything else in the world! So it had better be valuable!
Say no calmly and clearly, possibly using phrases like I’d love to but, I’m feeling a bit under pressure because of, So unfortunately I have to say Is that OK?
Don’t feel guilty they’ll be fine without you!
PS if you really can’t say no, then negotiating to do part of it, or to do it later, or for resources to help you, are other options you could consider.