My two cents:
The first and biggest mistake any guy could possibly make upon learning a girl has a crush on someone is getting pissed about it.
Why?
Well first, we have to remember that women don't see things the way we do. In fact, with women, a crush is little more than a coarse grain of sand on a piece of glass. Its there, but it isn't harmful, and soon the wind's going to blow it away.
But what happens when a guy gets pissed about it?
The same thing that happens when you rub something rough against glass. you make scratches.
There's nothing that makes a woman more attracted to a man than knowing she's attracted to someone who affects/intimidates/angers/whatever (insert word here) other men.
if you want to keep your girl, forget about this "crush" and treat it like what it is: "a very minor annoyance." Don't rub it in, or even mind it for that matter. forget about it. Just be you and make her see that she's not going to find a better man anywhere else.
Cheers, and good luck.