Probably one of the hardest things to do, but the most necessary in situations like that, is to encourage yourself. There's a Bible principle in the Old Testament of King David that in the lowest point of his life, when everybody was trying to kill him and even his own army of men was against him, David encouraged himself in the Lord. He asked his armor bearer to go get Goliath's sword. David was the one who killed Goliath. He remembered the battle that God had helped him win by looking at that sword and remembering that victory. That's part of looking at the big picture. You know you can have success. You've had it before.
It's so important to always keep your mind renewed to the right perspective. Don't be too low or too high. Work hard and earn the confidence you need.
The whole battle is learning how to fight the fight of thought management, and realizing that a battle will take place. True, tough competitors who learn how to win learn how to control all the aspects that can come into the mind game. By the time you think you're in a slump, you're fighting thoughts like "I can't win." Let those thoughts go.