Overpromising feels generous in the moment and looks like lying by the deadline.

Saying yes to more than you can deliver usually comes from wanting to be helpful, not from wanting to deceive. The person hearing it has no access to your intentions, though; they only have the gap between what you said and what arrived. Generosity that cannot be delivered gets filed as dishonesty, however kindly it was meant.

Subtract a third from whatever your instinct says you can manage, and promise that instead. Then deliver it early. If you are already over-committed, say so now rather than at the deadline, when the news costs them something.