The favor you cannot manage does not become manageable because someone asks more warmly.

Warmth in a request changes how it feels but not what it costs. A gracious ask, a compliment about how well you did it last time, a note about how much it would mean: all of that works on your reluctance to disappoint rather than on your actual capacity. The hours you do not have remain the hours you do not have. Charm and feasibility are separate questions.

Answer them separately on purpose. Ask whether you can genuinely do the thing before you consider how much you like the person asking. You can be warm in return and still say that it is not possible.