Well, the comforting thing (sort of) is that I've not met anyone's who doesn't suffer from this from time to time. I get it a lot, most often straight after a competition but also at random points during a project. As has already been mentioned, just take a break. Perhaps give yourself a few days to catch up on the film you keep meaning to watch, or an Xbox binge etc. Dont even look at the model during that time but set a date to go back and hopefully start again. You may even get twitchy before that time. But, as has also been said, paint something else. I find that painting some table top stuff helps or just experimenting on a non competition model. If you are really keen to get that model ready for the competition the time pressure will soon drive you on.