Some chess players down play the role that psychology plays in attempting to win a game of chess. Psychology plays an important role in not only your confidence but in the confidence of your opponent to win a game. This is particularly important if you are playing someone that you have lost to before. If you approach a game assuming that you will lose the game, then you will probably make a mistake which results in your opponent winning because of your lack of focus on the game. Conversely, if you are playing against someone who you have defeated a number of times, it is easy to lose focus and assume that you will win. Assuming that you will win will likely lead you into making foolish mistakes in the game.
If you are playing in a tournament against a higher rated opponent it may be that you feel out gunned. The most important thing that you can do to eliminate the psychology of the game is to play the board, not your opponent. Play against how the pieces are shaping up and what tactics and strategies your opponent appears to be executing. Try not to fall into the trap of believing that since your opponent is higher rated that you will lose. The truth is that any player can beat any other player if you are able to limit your mistakes and capitalize on your opponent's mistakes.
Do not doubt your abilities to win a game and do not doubt the abilities of your opponent to win if you are not focused on the game. There are many famous stories of young children shamelessly defeating a high-ranking player. Part of this was due to the high-ranking player assuming that a child could not possibly beat them. Play against the board, not your opponent's personality.