When I start to get nervous or lose my confidence, I begin to remind myself of the disgust I feel when I lose a match due to being tentative.
I also remind myself why I play the sport. For me, winning tentatively is just barely more satisfying than losing while playing well. However, sticking with aggressive play during pressure will do much more for your game than a tentative win will in my opinion.
Also check out 'The Inner Game of Tennis'. It's a good book about blocking out irrelevant and negative thoughts. I'm surprised no one mentioned it yet.