Very sad and depressed when not with my girlfriend. Why?

I finally found the perfect girl, she is everything i have always wanted. I am the happiest person ever when im with her but the second we are apart i am extremely sad, sometimes even depressed. she is all i think about and we have been dating for about 6 months. i love her with all my heart and she is my first love. the feelings are completely mutual and i am her first love. why do i feel so sad though :( so sad i cant even focus :(