How long ago was it, iteracom?
If it was a while back, you can write her a letter saying how you were remembering her and thankful for all the great times you had with her. Women love that stuff, especially if it’s on paper, so they can store it in a secret place and reread it whenever they feel down.
Maybe she will write back and tell you what is up in her life. If she’s single still, at some point you can tell her that you still think she’s wonderful and that since you last spoke you’ve grown and changed for the better and would like to be a stronger, better BF for her this time around. Assuming you didn’t leave her in a really nasty way, she’ll love that, because it’s the stuff of fairy tales and handsome princes, etc…
Personally, for me, I can’t say that any past GF is “the one that got away.” I know why I was with each one; I know why each one didn’t work." I sometimes regret “what almost could have been” (particularly with my most recent ex), but I’m always aware that what I miss is “what I think it could have been,” and not “what it actually was.” When I make sure to remember “what actually was,” it’s easy for me not to reminisce irresponsibly.
If you think “what it could be” is achievable, and repeating “what it was” is avoidable, it’s worth getting in touch.