Hey, man, I'm sorry to hear that. If she's leaving you to "have fun" then she just isn't ready to be in a relationship. A few months ago, I went through something very tough with a woman I'd been with for 10 years and a lot of people here on GFY tried to give me advice and comfort. Mostly, what everyone said was "Give it time." I know that doesn't help (trust me, I was just in your shoes) but it's the only answer. If she doesn't want to be with you, you can't force it.
SpaceAce