Disclaimer: Usual disclaimers apply. I don't own Harry Potter or the Harry Potter universe, of course.
Author's note: There is a trope in The Teachers' Lounge forum, of Snapoetry – the kind of emo poetry a yearning Snape would write for Lily. Unfortunately, I'm not good at emo poetry – my teen self wrote goofy doggerel. So here's some from Snape to Lily, circa 1976.
Forgive Me, Dearest Lily
Oh maiden fair, with flaming hair,
Do not reject your loving Prince!
For you can see, your destiny,
It lies with me, and no one since.
A word in haste (Oh! what a waste!)
Drove you from my worthless life.
And what I see when you're with he
It guts me like a potion knife.
Do not reject nor disrespect
My hope that you might yet forgive
My o'er-quick tongue, for one so young
As me has so long left to live!
And I can show, as you well know
That I can give what he cannot:
My whole heart is but a start
Compared him, that preening sot.