This song wasn't on any of the official track lists for season 3 of Stranger Things. It appears fifteen minutes into episode 2 when Mrs. Wheeler tries to talk to Billy at the pool house after he's been infected.
https://spaces.hightail.com/space/wr1dzR8mmm
The lyrics I can identify are:
"I've done it again
I've let you in
To tear my heart in two"