You clean your kitchen when it gets messy, you clean your car when the back seat gets cluttered, and you should clean your mind when it gets full of negative things. This isn't a question of ignoring real problems you may be mulling over, it's a question of letting go of worry. There are things you just can't do anything about -- those are the things you have to stop thinking about today. Move on with positive thoughts, and leave the old ones behind.