Love these threads.
Read Dr. Bob Rotella's "Golf Is Not A Game Of Perfect."
The basic gist of it is, to maximize tournament results, a player must work and focus on their mental game no less than any other aspect of their game.
As for stupid mistakes, that's part of your mental game. All players must accept that over an entire tournament, mistakes will happen. Shots will be dropped. Results improves once a player accepts a bad throw or a stupid mistake and moves on. Dwelling on bad throws or bad events in a tournament only lowers the odds of success on future throws.
A big Rotella concept is the importance of confidence, no matter how irrational it might feel. The idea is that any other kind of mental approach to a shot, aside from maximum confidence, only hurts the odds of a player executing a particular shot. It seems simplistic but it's amazing how many players allow negative thinking to occur while they play.
E.g., think about lining up a tournament putt that you really need. Usually you make this putt 50% of the time during practice. What are your odds of making that putt in that tournament if you're thinking, in any way, about possibly missing it? After all, according to your abilities, you should miss that putt 50% of the time. There's really no way to quantify those odds except to say that they're under your regular odds of 50%. Now, if you tell yourself that you're going to make the putt or if you simply decide that you will be making it, what will happen to the odds of making that putt? Again, there's no real way of exactly determining that except to say that they will be a lot closer to your expected odds of 50%.
Playing with Irrational confidence is the only way to play.