I Am Sad That One Of My Best Friends Is Leaving

She isn't my best friend, but she is leaving. It's not fair because she is the literal glue that holds our friend group together. One day she was gone, one day, and the group nearly died. She came back from being sick, and everything was fine. We are awkward around each other but we tease each other and crap. She's leaving and I'm sad!!!!! I don't want her to go! I'm supposed to leave, not her! I leave every two to three years, not my friends! This sucks. ;(