Beginners always have a problem psychologically dealing with higher rated players (Sometimes i do too) but the thing is that they are scared of a loss. And MANY MANY times when they come close to a win they become slightly overconfident and end up miscalculating moves.
The trick to be calm in chess is to think that once your opponents were beginners too.They are humans.They too, make errors.When you are close to winning your adreneline starts pumping. To overcome this take a deep breath and calculate your variations neatly.Rememer the hardest thing to do in chess is winning a won position.
In a loss do not go in a spiral downfall.Think about the neatest way you can get out of the trap.If you lose think about the experience that you have gained. Proceed with this in your mind while playing chess games.And remember practice makes a man perfect.(well, almost )