That depends…What is it that you keep thinking about and cant get it out of your head?
Usually it happens when you have missed something or left something like undone work behind.
If you face it again, or if you finish the work…than you can let it go.
Or learn to let it go…Its hard and takes few months of practice but it comes good in the end and benefits you.
My friend used to tell me to let things go and I’ll feel better when I let them go. It will help me concentrate more on hand situations and future planning than worrying what I’ve done in the past.