As you're playing in a tournament, changing your mind to "Hey, I can win this thing" is incredibly detrimental to your game because you are imbalancing yourself. You need to assess hand after hand, bet after bet, move after move and proceed stoically. You are fooling yourself and it's a natural problem. Don't worry, I had it when I started playing too.
For instance, once I started to think, "Hey, I'm going to lose this thing," what do I need to do to last longer? Well I started playing a tight aggressive game and work on hand after hand. By focusing on the little things, I last longer. By focusing on the big picture and try to dominate, you will lose every time.
I'm not saying don't believe in yourself, I'm saying believe in your ability to control your mind and not act emotionally. You can achieve this by learning patience. I would seek a self help book on how to do that because it is different for everyone. Some may disagree on this but another way to become a good poker player is to start reading dense philosophy. It will challenge your way of thinking and allow you to learn to think about your mind. In thinking about your mind, you will begin the early stages of learning to control it.