Here's a great way to begin to improve your state as you set up over a golf shot.
I always remember what Bob Rotella wrote in his book, "Golf is not a Game of Perfect." When talking to Fred Couples about what his routine was, Couples suggested that, over a shot, he would imagine the best shot that he'd ever hit with that same club. From this one statement came the idea of creating a golf shot journal.
I've done this sporadically over the past 10 years and have built a significant catalog of great shots for myself. Re-reading this from time to time reinforces the sense that I've hit great shots and I'm therefore more capable than I might believe.
Try it yourself... After every round, write down the 3 or 4 best shots that you hit that day. You may have to dig deep to find them but they'll be there. It might have been a great drive on a tough hole, a crisp iron shot onto a green, a chip in or a great par putt. It doesn't matter. What matters is how you write these down. Use as much information and emotion as you can to elaborate on the situation and the shot. Who were you with, what was the weather like, what did the shot look like, what did the ball do and, most important, how did you "feel" when you hit it? The more colorful the story, the more likely you are to be able to recall it when you need it next.
Part of the difference between the game's elite players and the rest of us, is the depth of their golf shot memory bank. They've hit so many great shots with all of their clubs that it's easier for them to recall them. For a struggling amateur it's very easy to find a poor shot to recall instead of a great one.
By consistently writing your shot journal, you'll begin to over write the "poor shot" catalog and begin to think more positively. For a head start, fill the first few pages with all the shots you do remember from your golfing life. You may even surprise yourself as to how many you've stored away.