Since I'm not a psychologist I will not try to get deeply into how the mind works during a game of golf -- who knows anyway? -- Everybody’s different. But whether you call it Golf Psychology, or Sport Psychology, or The Mental Game, The Mental Side of Golf, Trying to Get into the Zone, whatever you call it, from my experience the mental side of the game of golf really boils down to something simple:

The most important thing you can do mentally to give yourself the best possible chance of success is to focus your attention on what you want to have happen.

Yes, it is simple in essence. No, it is not always easy to do. Yes, there are distractions. No, it's not always a trivial matter to focus.

Example: You have a putt to win the Galactic Championship of Golf. Perhaps there are many things going through your mind. Do you focus your attention on thoughts of, "Oh &#%!, I'm putting to become the best golfer in the galaxy," or do you focus on the process of making the putt?

 
Example 2: (more realistic) You're on the first tee at your usual course on a weekend. There are numerous people in the vicinity and the first two players in your group have hit nice tee shots. Do you focus your attention on the fact that they hit good tee shots and what if you don't? Do you focus your attention on the fact that there are people watching you? Or do you focus your attention on the procedure of hitting your tee shot where you want it to go?

This is obviously easier said than done, but the fact is that you are the only one in control of where and how you focus your attention. If your attention is on something other than the immediate task, it seems unreasonable to expect a good result, doesn't it?
Having a comfortable pre-shot routine helps tremendously with focusing.