It just comes down to not acting on it.
Don't make the crush any bigger of a deal than it already is. You're infatuated with a 13 year old girl, yes, but it doesn't mean you're going to act on your feelings. Not every crush is acted on and not every crush leads to something... the worst you can do, at least in my opinion, is to dwell on it and make it something huge when it could be potentially harmless. You have self control and I know you're smart.
A lot of people have been posting the consequences of getting with this girl and I think you already know all that by now. I think people are blowing this out of proportion because unless you are actually thinking on acting on it, I see no real issue. The crush could be gone in a few weeks or a few months, but by dwelling on it and actually blowing this thing up even more is going to make it even harder to deal with and make it even longer to get over.
I know it's hard not to think about it, but try not to pay attention to it.