I understand that people get angry for playing badly, and I do get angry too at times, but sometimes I wonder if the "playing badly" evaluation is just an illusion that we use as a self-defense mechanism. If you think of it, the "better you" that you should have been doesn't really exist. If you play well while practicing, but choke in competition, it's not fair to say that you didn't play to your level, because your level is whatever you are able to produce in the competition. If you feel like there's a big mismatch with your practice play and competitive play, then you have some mental issues.
If the level of your play varies significantly from match to match, from tournament to tournament, that again is a flaw in your game. There's something that makes it difficult for you to play consistently and that's something you should work on. You shouldn't think that your true level is the play you can produce when you're at your best. Your true level is an average of all your playing levels.
Also, it's likely that you overestimate your abilities anyhow. That's a basic tendency for us humans. We tend to give less weight to our bad days and more credit to better ones. I'd guess that majority of players think they're better players than they really are. I certainly do.
