I learned piano, but I don't play anymore. I've also learned (and stopped) Trombone and Sax. I somewhat learned guitar, but I don't get the chance to play much, though I still build guitars(electric and cigarbox) on occasion. I also drum a bit, but with no kit and nowhere to practice anymore, I fear I'm getting terrible rusty.
Friend of mine learned Violin, she still owns one, but I don't think she plays very much at all. A few friends didn't get far beyond the "aspiring" part of aspiring guitarist. My sister tried to learn drums and gave up, ditto guitar - in fact, that's why I picked them up, we just had instruments sitting about. I know one bloke who played trumpet right up until I think he was graduating college, and he basically had to make the choice between full-time professional musician, or IT professional, so he chose the latter.
I do have one friend, though, she learned Viola, and never stopped. Was in the local youth orchestra, the city and state symphony orchestras, and nowdays she's still a professional musician and music teacher(Voice, Viola, Violin, and Theory), with two private studios for lessons, and a very active career. I'm incredibly proud of her.
Also, fastest way to find out if someone plays a viola, ask them if they play the violin. The Violin player will say yes, the Viola player will batter you with a rosin block.
And for all of you who are saying "Well, I stopped" or "I'm so sorry I didn't continue with it" there's also my father. He used to be able to play one song, poorly.
And that's how it stayed for twenty fucking years, and yes, because you asked nicely, I still can't listen to Sloop John B by the Beach boys without wanting to strangle someone.
But about six years ago, after a little prodding, he went "Right, fuck this, I'm going to actually learn", and he sat down, and practiced. And it sounded like shit, even though thank fuck it wasn't Sloop John B. But then it sounded less like shit. And a little less. Then it started sounding alright. Then really good. By the time he was 65, he's playing well enough that Phil Emmanuel is telling him he's a pretty good guitarist.
So don't give up. You can still learn. Just get back on the horse, and play till you've got blistas on ya fingas.