“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.
This ex was a good person, your breakup didn’t change your opinion about this.
They are someone who deserves love in their lives, someone another person would be lucky to be with. They had things they needed to work on, but then again, so did you. You’re both good people, but maybe you just weren’t good enough for each other at the time.
The way to spot “the one that got away” is to think of the ex that makes you hopeful about what your future love life can be like, rather than making you upset or pessimistic about human nature. You know your ex — this ex — will always be the one that got away because things will always be unresolved. You still respect them too much to let your breakup tarnish your opinion of them. You know the person you end up with is going to have to do better — the one that got away always raises the bar on what love can really be like.