Saying yes becomes expensive when too many good things compete for one life.

Refusing bad things is easy. What costs you is the collision of genuinely good options inside one finite life, where each yes is defensible and the total is not survivable. That is why capable people end up overloaded, not by weak judgment but by too many correct individual decisions.

Stop evaluating requests one at a time against whether they are worthwhile. Evaluate them against what is already committed. The honest question is not whether this is good, but what it displaces and whether you would trade.