Psychopomp's Song reads "When you would take 1 or more damage: Take 2 additional damage..." This means that this effect is essentially modifying the amount of damage you were about to take (but have not yet taken, as this is a "when..." effect). If you were about to take 2 damage and then Psychopomp's Song caused you to take 2 additional damage, you could cancel all 4 of it with "I've had worse…".