When set this type of challenge, most people find it difficult. The reason for this difficulty is that trying to move thoughts from the conscious mind to the unconscious mind is an effortful process.
If you have worries that are interfering with your day, postpone them until a later time or date. You don't need to suppress the thoughts; rather you are acknowledging them but postponing how to deal with them until later. Set a time to ruminate or worry. This will allow you to take control and also may reduce the anxiety and depression associated with your worries.
(Borkovec et al 1983b, 'Stimulus control applications to the treatment of worry'.)