It has been a long time since I have read this book but I would say the moral of the Mirror of Erised is to not dwell on the past. To not focus on what you do not have and instead to focus on what you do have and make the most of it. Harry was so intrigued by seeing his family in the mirror that he no longer accepted that he was an orphan and Dum bledore had to put an end to it. Instead of dwelling on desires, make the most of what you already have.
Hope this helps! If you need any more help or further explanation just let me know! :)
