"Ler Her Go" by Passenger.
It was in heavy rotation on the radio as my wife was nearing the end of her life. She was diagnosed with pancreatic cancer, and in tons of pain. It only lasted a few weeks, but as she pressed the button for the pain meds, she would often cry and beg "Just let me go".

It brings back the feelings of helplessness from that time, and the loss I still feel in my life.

On the positive side, "Rock Lobster" by the B-52s. Me and a few friends finally managed to get into CBGBs (we were very underage), and caught them when Rock Lobster was just a single in '78. We had a great time, and I am still in touch with most of those friends today.