Yogi Berra said famously, "Baseball is 90% mental, the other half is physical".
As softballphreak said, there are tons of books written on the subject.
The best players aren't the ones that take ground balls the best or have the prettiest swings, they are the ones who win in their heads every time they step on the field.
Positive mentality (can do attitude) breeds confidence, confidence in every situation makes you comfortable, being confident and comfortable in pressure situations leads to success. Success leads to confidence...
Negative mentality does exact opposite.
Learn to controll your mind and you will be a success at anything you want bad enough to work for. Easier said than done, but when you realize you can, you can disregard all the negativity you are bombarded with by naysayers and you will be able to controll your own destiny in whatever you want to do.