Warning: I am no expert when it comes to writing poetry. So bare with me...R&R pleeeeeeeeease!
Disclaimer: J.K. is queen...

Severus Snape

I watched...
You playing near Spinner's End.
To you hidden kisses, I did send.
That all stopped with your sister's frown,
And when I sent that tree branch crashing down.

I watched...
You come into your wizarding own.
Little did you know how my affection had grown.
Honoured to be the one you spent all your time with,
But for what I did next, I would never forgive.

I watched...
Your look of disdain when I uttered the word.
My apology seemed forever unheard.
James Potter was now your beau of choice.
Oh, how I wish that I had found my voice.

I watched...
As the Dark Lord searched for you,
And my inward anxiety grew.
You died in the name of protecting your son,
And all I could think was 'What have I done?'

I watched...
Out for your child on Dumbledore's orders,
though my hatred of Potter could never be altered.
My feelings for you remain passionate, still.
Lily Evans, I love you, and I always will.