My favorite metaphor to play around with is that thoughts are like actual trains. You can watch them go by and pick which one to get on.
Are you going to pick the stress train or the mellow train? Are you going to pick the happy memory train or the depressing memory train? Are you getting on the self-blame train or the self-acceptance one? You have a choice. If you get on a negative train of thought, you can always get off at the next platform and switch trains.
A mentor of mine told me that meditation is just sitting on the platform, watching all the trains go by, and not boarding one at all. When in doubt of which train to pick, wait it out at the platform...
Which thought trains have you been on recently?