Bro for some guys feeling the way you do is common. Do whatever you have to do to turn those negative feelings into positive actions. This might sound gay, but go out and buy some new clothes, go tanning, head to the gym and rip out a sick workout, hang out with friends and talk with other chicks. Do whatever so build your confidence and pride back up which can get destroyed after a break up.
Shit, your on a cycle right now man, the gym should be a party for you.
Girls that will up and leave you at any time are a dime a dozen and you shouldn't worry if they leave anyway. You'll find another girl and forget all about this currrent one.