You don't. Hold it all in, and let your jealousy grow as you see here with other people. Eventually, she'll start a serious relationship with someone and you can direct your hatred towards that person.
Still, you need to keep silent. Don't tell anyone and allow yourself to sink into insanity, but continue with your regular life. Then, when you finally lose control, find where they are living together and torture him before her. That's where she'll understand to what lengths you are willing to go to for her.
What I meant to say was tell her soon or somebody else will, and when that happens, you will regret it.
There are two paths you can take if she does wind up with someone else. You either get over it, or don't. If you don't, you'll either do something to split them apart, or become psychotic. Either way, it will probably end badly.