I know what you mean. Hypersensitivity to rejection is horrible. I just lost my best friend a few months ago.

Maybe if you were to tell yourself that your life and her life are two different objects in the universe. Yes there is a connection, but that doesn't mean they are the same thing. If you were to be separated, it would not be the end.
And also, as I'm sure you know, the more you worry about it the more worried you get. Maybe, every time a thought comes of "what if (s)he leaves", replace it with a memory of a good time you've had together.
Good luck!