I fully understand your feelings, but a true friendship can survive distance.
It will not be the same as having her next door, obviously, but you can stay in touch and still share dreams and feelings.
My best friend lives in England and I live in Germany.
When I left England she might have felt the same way you do now.
She said: Don't go, what am I going to do without you?
That made me cry, but I still had to go.
Today I see her once or twice a year, call her once or twice a month and usually text her once or twice a week.
The relationship has changed.
But her life continued, and so did mine.
You will meet many more people during your life.
Give them the chance to become friends, too, and be happy for your best friend if she is happy enough to have a goal in life.