Whenever I make a misplay, I make a note of what deck I'm playing, what deck my opponent is playing, what I did, and think about that whenever I play the same matchup again. For example, at a small practice regional tourney, I played Plasma against an Yveltal deck. I literally had the win (discard my hand to get the 4th deoxys for the KO), but mistakenly played down a thundurus, giving my opponent the win by me only dealing 160 damage to a full HP Yveltal. This mistake now comes into my mind whenever I play a deck that revolves around benched pokemon (Eg. Empoleon, Raichu, etc.).
I also want to point out that through the course of the day, I make sure that a loss or a big misplay doesn't affect my performance later on. I do this by setting little goals after each round. Lets say I'm going into a 9 round regionals, and I lose the 1st round, I'll say 'well I can at least go 8-1 if I win out', and then later on after the 4th round I'll say 'I can still maintain a positive record at 5-4 if I win out', etc. Each time I win, lose, or tie, I try to make the outcome as productive as possible, from going to top cut, to top 32, to prizes, then to a positive record. In general it keeps morale up and can even improve your games when its 4pm and you still haven't eaten.
Lastly, mistakes happen to everyone. I recently at a cities forgot prize cards AND used 2 supporters in one game (which I just conceded at that point because I felt as if I didn't deserve to win the match-up after 2 major misplays), yet I still am a very good player. I've talked to many Worlds players, and even in this simplified format mistakes can slip by anyone, and if you watch enough streams I guarantee you that 1/3 rounds need to have some kind of judge ruling on something. So don't fret it, just think about your next matches and hope to win out!