What everybody has said is good... I would also say having good confidence, confidence gives you energy, focus, concentration and lot of benefits that you wouldn't have just by being insecure...... I think many people use the expresion "holy snap, this guy has exelent concentration in such and such" .... But what he actually has is a lot of belief that he can do whatever it is which breeds a calmness and focus and positive feeling...... basically my point is confidence overlaps many areas of your mentality and if you have it most of your mental skills will be mighty sharp, if you don't you'll be a little slower, a little less concentrated, a little more doubtful, etc...
A little off topic, but it has helped me tremendously and my stats and game's have trippled.... I read up an old soccer4life guide, on mental preparation before a game.......I've been trying the yelling part of the guide, before the games, and its gave me the best killer instinct and confidence boost on the field ever.... If you can give it a try, because it surely takes your confidence to another level, its a temporary boost, like for the duration of the game, but you'll play a mighty good game if you do it... :
1.2 Yelling. This is similar to questioning yourself, however without the questions. You can vent on the other team, your little brother, etc. It doen't have to be soccer related, just something to get you mad. You want your veins pulsing, and adhrenaline pumping. Vent, vent, vent; rant, rant, rant. Even if you do some non-soccer related yelling, probably add some soccer shouts, too, talking about anything and everything, but maily what you are going to do, and how you are going to accomplish that.
(Ex. This is my game! I can do this! I will do this! Nothing can stop me now! I'm at the top of my game! This is everything I have ever worked for! I'm going to give it all I got! I'm going to dominate! This is my time to shine! I won't let anyone take this from me! I deserve this for myself and to my teammates! Everyone will want to be me! I'm going to be the best!)