It certainly is not "utterly rubbish", don't even think of such a thing! It is believeable, indeed. But I think it can be improved. You see, you're attempting to write one of the most emotional and trumatic scenes in HP-verse. I think your fic is really good, but it's really short for such an intense position. At this stage, it looks like the skeleton of your real story. There are lots of thoughts and feelings and emotion that can be filled in between the lines. For example, the beginning is great: Snape breaks into a run, in the shock of understanding his fault. That part's very good, I think. The rest of the story could be "filled in" as such, if you know what I mean.
So, overall, I think it's a very sound and solid foundation on which you can definitely build miracles.
Author's Response: Thank you for your comments :) I know it's a bit short but I wasn't sure what to write if you know what I mean and sometimes it can be more effective to write less...but I dunno lol thanks