I broke the quest "Red, Then Dead"

In Borderlands: The Pre-Sequel there is a quest called “Red, Then Dead” where you are tasked with killing three Lost Legion couriers and obtaining their ECHO messages. There are mission requirements for both killing the three couriers and collecting the three ECHO logs.

What happened in my game was…
As I was approaching the first courier, I noticed that the second courier had already spawned (the first two are really close to each other). So, for ■■■■■ and giggles, I killed the second courier first. Then I killed the first courier and picked up his ECHO log. This caused the second courier to respawn. So I killed him again and proceeded to pick up his ECHO log… Except there were two ECHO logs on the ground here, because I had killed the second courier twice. And the other ECHO was actually the one you’re supposed to get from the third courier.

Picking up the third ECHO, without actually having killed the third courier, completely messed up the quest.

Now I can’t complete the quest because I have collected all three ECHO messages and the mission is no longer prompting me to kill the third courier. But I also can’t turn the mission in because I haven’t killed the third guy. Quitting and restarting the game doesn’t help.

The completionist in me is crying.
And the grown-up in me is offering the most insincere consolation: “There, there…”

Still, maybe you guys would want to know that players can lock themselves out of the quest completion here.