what to do?

Sometimes people go through phases where they are thinking about other people. In order to answer your question properly, I’ll need to know what kind of thoughts you have of this person exactly. I will say though, that in the majority of cases it’s just curiosity and it passes with time.

Think about why it is that you think about this other person and whether or not certain thoughts of this person have been a result of minor failures within your relationship (does happen). Consider your relationship and how you feel towards your boyfriend as well as what your history was with this other guy. Search within yourself and identify why it is that this person is on your mind and what it is about him that doesn’t allow you to forget about him. Any other information you could provide could assist me in coming up with a more precise answer.