We have a few good videos in the psychological video section on this type of thing on the Academy so definitely worth checking those out!
The main thing I would say is having a game plan, structure or tactics to follow to keep your mind clear and inly focusing on a few key points that you need to tey to execute against that particular player. This will keep you more focused and not so stressed about each individual point. When things seem to go down hill fast, make a real effort to slow the game down and take your towel when possible to break the opponents momentum and also give you an opportunity to calm down and re-focus.
