The one that got away” is an ex who has an exalted place among your past loves. They are the one you focus about. They are the one who floats to the surface of your thoughts when you are trying to sleep, the one you can still picture a future with — in a parallel universe. You just aren’t sure whether it’s the universe you’re supposed to exist in or whether you belong right here in reality.
The one that got away isn’t someone with whom you had a terrible, devastating breakup. They didn’t cheat on you (or vice versa). Things simply didn’t work out because you were young and immature, or one of you had to move, or you just had other things you needed to do before you found this kind of love. The point is that things never ran their natural course. You never really found out that you two could never work out. It just ended, for circumstances other than falling out of love.