Geez! You're already talking about her leaving at the end of summer and April hasn't even come to a close yet! You and your girlfriend literally have loads of time left together. In that time who knows what could happen. Your relationship could grow and grow.
I agree with Hawk, why do you have to end it when she leaves? A lot of people have successfully maintained a long-distance relationship.
As far as bi-polar goes, you can find a lot of useful information in the "Depression" forum (bi-polar is a form of depression).
Honestly I wouldn't start worrying just yet about your girlfriend leaving. The two of you have a whole summer ahead of you.
Nicola