“I can’t tonight” doesn’t need a paragraph of reasons. Good friends accept short answers.

A long justification is usually there to prove you are allowed to decline, which assumes you were not. Piling on reasons also invites negotiation, because each one can be solved, and now you are defending a decision you already made. The people who actually care about you will take a plain no without an audit.

Decline one thing this week in a single sentence and resist adding the because. See who is fine with it, since that tells you something useful about them. And when somebody gives you a short no, take it the same way rather than pressing for the story.