I think oftentimes we think that another guy/girl is the reason someone initiates a breakup. In reality, the person who ended it probably wanted to end it for a while, but waited until they had someone else lined up. If you are the only person she's ever been with and you guys have been together that long, then it's probably scary as hell for her to think that you might not actually be the one.
The only advice I would give if you want to try to save your relationship is to pursue her new interests with her. She may is growing as an individual and doesn't see you fitting into that. If I were you, I would offer to join her in going out, going to the gym, etc, and grow with her.