Plug too many things into one outlet and the whole thing trips. Same with commitments.

Capacity is not a matter of willpower. There is a real limit, and when you exceed it the failure is not graceful, it is everything going dark at once. Each thing you said yes to was reasonable by itself. The load is what breaks the circuit, not any single item on it.

Count what is currently plugged in, honestly, including the things you do not think of as commitments. Then take one off before you add the next. If nothing can come off, the answer to the next request is no, and no is a complete sentence.