that "i got him, haha" feeling that quickly turns into "why the F isn't he resigning" is what gets you off-guard and makes your luck flip (and you lose, then come here to biatch about players not resigning after they blunder that first piece).
That feeling when you have big advantage but opponents outplays you

... cause, after you blunder those first few pieces, there are 4 possible outcomes of such game, 3 of which are in your favor: you lose, you draw/stalemate, you win. you, therefore have 75% of pulling out a better outcome then what may be obvious to the princess that you're playing against (that is, to resign). so, it's quire silly to resign, instead of using that opportunity to try to learn how to navigate out of tough situations. never give up. that drives a percentage of chess-snobs crazy, which makes them relax too much and, in fact, lose.

I've been on both sides of that...it's great when you hang in and grab a win through sheer bloody mindedness

Nothing more pleasing than being aggravating at this game
Until you get rematched and get your ass kicked

1200 player talks about blundering to an 1800 player... lol
Have done it many times vs lower rated than me. But now i have experienced it against higher rated probably FM...
Oh man.. you got him.. you are up plus 6 or close. But you lack the technick. And you know it. Its like hitting a wall. Solid rock. You know you have a win and just couple moves and victory is there. But you cant fucking find those moves are invisible to you.
Nothing from ordinery practice or principles helps here.
This is advanced stuff and only if you have 10 minutes to think you could find them, but impossible when you have 2 minutes left. And yet your superior opponent finds the defense or attack and beats you...
I just cand do it. Too complicated.