So, I found this Snape/Sinistra (and also Snape/Lily, sort of) fic that takes place around Snape's "death," called I Promise. In case you didn't know, I put quotes around that because he doesn't die in this.
To be honest, I really hate the author's interpretation of the Lily/Snape relationship, but I'm late in posting this as it is, so I wanted to get it over with.
It's two chapters that are of medium length, and even though I didn't like the Lily/Snape part, I still like the fic. Oh--and it's written in Snape's POV and in present tense.
Expecto Patronum!
Silvery gas comes out your wand, and forms to shape this link: http://www.fanfiction.net/s/7180396/1/I_Promise