What a lovely comment to wake up to this morning - thank you! I've only been writing Snape for a short time, but I find that I keep veering back in the direction of digging through his psyche to understand both his horrible-ness and the things that make him so appealing. I think maybe you've hit the nail on the head - he's so wonderfully flawed, and in such a human, relatable way. 'My' Snape, so far, is never quite as shrill as in canon - I think, in part, because I just can't reconcile that shrillness with the other aspects of his personality we're given. It's as you say, he knows his weakness - knows it for what it is, but also what he gets from it.
But I'm babbling. :)
I'm actually working on a pt. 2 to this right now that's pretty much all Snape, although I'm not sure it will please. I've basically decided that he's going to have to know himself even better, and make hard choices about what to cling to and what to let go of, if he's going to grow in any way. Rightly or wrongly, I want to save him and give him the life he deserves after everything. Potential for crap runs high, but I'm giving it a shot. :)
Thank you again for this wonderful comment - it made my day.