When registering for such a tournament, you need to force yourself, motivate yourself, and try. You can always find excuses everywhere, but forcing yourself to play when you don't feel like it is what defines a good player. After all, no one forces you to play; you register for the competition yourself. And if something unpleasant, bad, or unfortunate happens, you can always tell the truth or politely withdraw due to a lack of motivation, and people will understand.