Catching Fire - Rubbing salt in a wound

“Rubbing salt in a wound. For the first time I truly appreciate the expression, because the salt in the water makes the pain of my wounds so blinding (...)"

Rub salt into/in the wound

Similar to "Echarle sal a la herida". To make a difficult situation even worse for someone. 

Losing was bad enough, having to watch them receiving the trophy just rubbed salt into the wound

Rub:  apply firm pressure to the surface of (something), using a repeated back and forth motion

Wound: an injury to living tissue caused by a cut, blow, or other impact, typically one in which the skin is cut or broken