Which object is extremely useless, but you just can‘t get rid of it because it reminds you of something?

A stone, me and a friend of mine Jerusha, both went on holiday around the same time. I went to Austria and she went to Italy. As a joke we promised each other to find a stone in the countries we went to and exchange them. She remember, I forgot. Its become a reminder of guilt sitting at the end of my desk looking at me as I’m typing this.

Antonia, 23