It is natural to still recall good times with an ex and think about them fondly sometimes. But I think the best definition of being "over" someone is where you are in your life. The desire to get into a relationship with someone else and the desire to make it work with the new person. Not a rebound, though.
In many ways, breaking up is similar to the grieving process. You have to work through the stages before you come out the other side. And everyone grieves a little differently.
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