It's a good question. I think if you start to think "DON'T DROP IT!" then every time the ball comes near, it can make you tense up and even worse at catching. However, if you "don't worry about it", you might not care enough to stop dropping catches (but at least you'll be able to deal with it

).
You need to clear your head, but not too much. As much as you should care, it's no good dwelling on the failure, or what it cost, because you can't rectify it. You can only attempt to field better next time and moping about is only a hindrance to that.
If you think about a past or practice example of the catch, you can start to understand why you failed and hopefully remember that you actually are capable of taking the catch correctly. Were you slow on your feet? Did you watch it all the way? Are you grabbing at the ball with hard hands? It's usually simple enough, but as simple and easy as it all is, it's even simpler to not do something. By imagining yourself doing it the right way, it can help you to concentrate for the next one, as well as possibly helping to understand your mistakes and weaknesses.
But to "get yourself up", you should find something to be determined about (ideally, something that you can't really fail on until after the match.

).
You can not only think about how you practice, but also think about how you're going to do a few more catches next time. If you resolve to practice harder, then you're committing yourself to catching better. Setting a flexible goal like this will help to be positive and confident rather than frozen with fear every time the ball comes by.