Philosopher and psychoanalyst Alan Watts described life experience as being like a ship: Events, like boats, leave waves behind them, but they don’t drive the ship — and eventually the wave fades and is gone forever.

People who are preoccupied with the past are often unhappy. While most of us have had difficult or even devastating experiences, little good comes from dwelling on them. Why? Because we are helpless to change the outcome.

How can we let go of a past experience that haunts us? Sometimes, as in the case of a trauma, we go over it in our minds repeatedly. A trauma is usually an experience that is so outside our realm of ordinary experience that it needs to be mentally or even verbally repeated to be “metabolized” into our personal narrative or self-image.

Freud conceptualized emotional energy as limited, and used the term “cathexis” to describe the investment we make in an experience or person. He used the metaphor of troop movements to illustrate. If you are in love with A, you are “cathected” to him and have deployed most, if not all of your troops to A. If you break up and want to move on, you must recall the troops home before you can redeploy them elsewhere.

This explains why it can be so difficult to get over someone if you keep seeing that person after the break-up. There needs to be some time apart for you to withdraw your cathexis. This can be hard if you are still in contact, though later on, once most of the troops are brought home, it can be possible to see each other as friends.

Using this metaphor, you can see how revisiting an experience can keep you from moving on. Thinking about the past cannot help—unless we use the technique of reframing—changing the way we think about it.