I find that when playing Cho'Gath I often "Overguess" when placing rupture in teamfights. As in, I'll try to do some goofy thing where I basically overthink where to place rupture, when instead the simpler and obvious answer was correct. I get this problem with skillshots sometimes.
How do I shut my stupid brain up?
Edit: Oh, it's you. Hi!
Save up your skillshots until enemies' options are limited (they only have one good path, they're stunned/heavily slowed, distracted by a fight, etc.) before using them.