Break ups are tough, I know of the place you're currently in.. yet my advice may seem a bit harsh, so be forewarned.
No one should have to shoulder the burden of being someone's entire world.. it isn't fair to either party. You feel as if nothing matters, that somehow your worth has been found wanting. Brooding will not help in the slightest, trust me. I felt that if this one person didn't like me then I have no chance whatsoever.. which isn't true. The world is too large a place to be that lonely. I'm not discounting your troubles in the slightest.. I'm about the same age and also have difficulty finding new social groups. You have to keep trying, though.
For (probably a dumb) example, you have an empty pitcher. You are tasked with filling this pitcher with happiness.. which is a depleting resource and must be refreshed from time to time. The wise thing to do would be to fill it up from multiple sources, so their eventual draining will only take a minor cut out of the pitcher. I can't answer what these sources will be.. things that offer you some sort of fulfillment and accomplishment. Hiking, reading, writing, drawing, playing music or listening to it.. hobbies that bring you joy even when alone. If you filled up said pitcher with liquid from one single source and that drains out.. you're in a rough spot.
(This is probably dragging on far too long so..)
Find happiness without other people. If she wants to go her own way, let her. Some things are beyond our control, no matter how hard we hope or strain. Keep moving forward, please. It sounds terribly cliche but.. someone out there, on this gigantic biosphere we call home, will understand you. Will cherish your company. Be it friend or something else, you won't find them stuck in a hole. Dig out of it and just.. keep.. looking.