Longing and Attachment, longing is the mind's response to the absence of something that once had a place in our lives; attachment begins when someone's presence becomes tied to our sense of security, peace, or emotional stability. That's why you can miss someone without being dependent on them, while with attachment, their absence can create not only sadness, but also a sense of insecurity and emptiness.
Longing doesn't necessarily mean wanting someone back, but attachment can create the feeling that "I can't be okay without them." The difference is simple: longing shows that something held meaning for us; attachment shows that we tied part of our stability to it.