"Dumbledore. There is something you should know. Sirius is an anigamus."
These are the words you did not say. And you hate yourself a little more for not saying them.
Everything worked out for the best though, right? You know Sirius would have been caught and the Dementors would have performed the Kiss. Sirius would be dead. But you didn't know Sirius was innocent when you decided to be silent on the issue, did you? All you knew was that you did not want to tell Albus that you had failed him. You think you're selfish for this, don't you?
You lied to him. Not directly of course, but through omission. Someone could have been hurt. Harry could have been killed. You were silent on the subject and now you are full of guilt. Why can't you let it go now that everything has worked out for the best and you have your friend back?
Because you think it was your cowardiceness that almost got everyone killed. Because you always thought you were better than that. But when you were tested, you feel as if you failed. You cant even be mad about what happened that night. You're just upset about why. You think the whole thing happened because of you. Because of your weakness. And because of your silence.