Glitter is the herpes of craft supplies.

First off, you're letting jealousy consume you. From experience, this is the absolute worst thing you can do, as once you fall into a jealousy rut it is extremely hard to climb back out on your own.

So you lost one. There will be another. I don't mean to sound cold; I in fact mean to be encouraging. And hey, look at it this way - he'll most likely break up with her, as high school relationships don't tend to last, and now he knows you have something for him.