After all, it was someone you once loved, and maybe you still can't get over it in some ways. For example, what did he do to you, or what did you do to him? You just want to sink into the memory of the past, and you still have two yards to love him. You don't have to mind. But don't let your current boyfriend know, he will be very angry.
You just dwell on past memories, which have nothing to do with your ex-boyfriend in real life. You don't need to feel uneasy or sad about your current boyfriend. You didn't betray him.