Enter a fight with full confidence that you've worked as hard as you could have in preperation for a fight! Take that confidence and use it as a "positive" reinforcement to you entering the ring. Visualize you entering a fight when your in the gym training! (spar, shadow, rope, bag work, exercise)
The fight is the finality of your training sessions. If you train all out then your ready to fight!
Confidence is developed in the gym for the fight. As time goes on and you gain more experience the fights should give confidence. Train hard to be able to fight hard!