Callous

A guitar.
The first time you're plucking at strings, you don't know what you're doing; but you're trying to make something beautiful- From the heart, that you'll understand.
And someone takes initiative to show you, this is "C", this is "A"
Fresh fingers are cut, and there's pain, but everyone says it's worth it.
So set it down, heal a bit, and come back again.
Remember a chord or two, a little diddy, but change it all around, and teach yourself new hardships, muscle memory, make a little progress.
And though your fingers are burning, you tell yourself it's worth the pain.
Put it down, heal a bit, and come back to it all the same.
And the process rolls on and on, until one day you realize you're playing your song.
And you say that it's been worth it.
But by then, you can't even feel, since once fresh, soft skin has turned to callous.
That is your sacrifice.