I can only equate this to "I'm never drinking again" after a night of hard drinking and an even harder morning after. You're depressed now, as you would be expected to be. You think she's the only one for you, because she's pretty much the only thing you know at this point in your life. But I assure you that once you allow yourself to move on (I realize that it isn't always so simple, you can see my topic down the page for evidence of that), you'll eventually find someone new that will grab your attention, and make you forget that you ever said this.
As for advice, the only thing I can tell you is to follow a little saying. I'll probably butcher it, but I'm sure you'll recognize it and get the point. "If you love something enough, set it free. If it returns to you then you know its real." Again, I probably butchered that horribly, but I'm going off the top of my head.