An excuse is an apology you drafted before you even let anyone down.

An excuse prepared in advance tells you the decision has already been made. You are not intending to do the thing, you are arranging the explanation, which means the disappointment is scheduled and only the announcement is pending. That pre-writing is worth catching, because it is the clearest signal of a commitment you never really took.

When you notice yourself drafting a reason, stop and choose properly: either do the thing or say now that you will not. Both are honest. The version that damages trust is the one where the reason arrives at the last minute.