I'm sorry to hear that. It's sad to see how people will stay in something longer than necessary until they are completely detached before giving you the boot. It's a very immature thing to do and is not being honest with your feelings in a way. It's not your fault that it took so long, it was actually very mature in a way. You'd be in an even deeper pile of shit if you guys were already married and this happened.
It sounds like she probably just wants to see what else is out there. She's only known you for this long, long time and probably wants to try something new and was bored with what she had already. Is there a little more backstory to this? I think understanding where and when things went wrong can help you in this healing process, though I'm sure it's difficult to look back.
Waking up next to a beautiful girl,
Step outside and say hello to my beautiful world.