I'm sure I'm not the only one who's got more than halfway into a run with good time and then done something stupid and ruined it.

What do you do when that happens?

I generally throw my controller at my bed behind me. Then I take some deep breaths and go for a walk.

Cry DendiFace

I laugh(?) and maybe cry Kappa

Well i just go play another game or watch some TV, videos or streams to chill out.

Besides crying like everyone else, I usually just retry that specific part for a while to practice and then I start over once I feel like I have a better handle on things. If I'm frustrated enough I'll just play something else.

I just continue to see if I can bestseg later splits OpieOP

Be pissed and mainly stop streaming and deal with some "RL" stuff..
Usually doesn't destroy my mood that much for some reason^^

"Pinky we must prepare for tonight"
"egad Brain are we going to raid someone on Twitch and spam NARF NARF"
"no Pinky! we do the same thing we do every night. try to get TEH URN"

Go for a walk or watch anime/Youtube, usually.

I go cry myself to sleep

Try to laugh it off. I used to instinctively slap myself in the face until I goofed up a Kirby's Dream Land run in a marathon once. Then I slapped myself. In the eye. And couldn't see straight for the rest of the run...

Either reset and try again, or play something else.

"I'm sure I'm not the only one who's got more than halfway into a run with good time and then done something stupid and ruined it."

I'm pretty sure you are the only one who's gotten more than halfway into a good run and then it died. Kappa b

^
Not always. Some games ¤Cough¤ New n Tasty ¤Cough¤ can have runs die at the halfway+ point because of either a glitch >.> or a crucial trick that u cant get.

B... B... But what about last-second-softlock in Half-Life 2: Episode Two??? BibleThump That's over halfway into a run.

Don't do anything (screaming, shouting etc.) and let that feeling of losing something sink in, once it settles take a deep breath (if tears are coming for some reason just let it tear) and decide if you want to reset or just stop for a bit. Of course this won't work for all good runs, probably depending on how good it was.