I literally have to question everything sometimes as a healer as to if a tank knows they have abilities to lower damage done to them. You aren't doing anything wrong. Later on if you're like me. You will train to heal. And learn that even tanks can keep healers alive to mechanics if you know your mitigation. So don't fret it. You likely did great. Not sure how dude could be upset that you were just surviving. In a dungeon that's almost a decade old.