It's so great to think about the past and smile and have that fuzzy feeling. That's healthy!
Sometimes, however, we stay in the lingering thought of the past and feel an emptiness or real longing to be back there. Sometimes those feelings don't allow us to stay in the present in our minds or in our hearts. Meaning we carry the pain or longing as though it belongs in the here and now.
If you were to stop and really realize that in the present all is well and separate yourself from "what happened" in the past, then the burden would be lifted and all would be well.