First and fore-most, you must have confidence. Your second mental problem is concentration. Think the shot through in advance before you address the ball. Draw a mental image of where you want it to go and then eliminate everything else from your mind, except how you are going to get the ball into that preferred spot.
Sam SneadI shot a wild elephant in Africa thirty yards from me, and it didn't hit the ground until it was right at my feet. I wasn't a bit scared. But a four foot putt scares me to death.
Sam SneadTo quit between tee and green is more habit-forming than drinking a highball before breakfast.
Sam Snead