Saying no to a friend once is how you say yes to them for real later.

A yes that was never optional is not worth much to anybody. Once a friend knows you are able to decline, your agreement stops being automatic and starts carrying weight. Refusing once teaches them the difference. It also spares you the slow resentment that builds when every request gets the same tired answer.

The next time you genuinely cannot do something, say so plainly and without a story attached. Then show up fully for the thing you do agree to, so the pattern is obvious.