I was looking for this same story today when I ran into this forum post. While a year late I hope this answer helps you. The story is still up. It's called "Imaginary Friend" by Lunarwench. Happy re-reading!
Funny you mentioned it, I was just reading it off my kindle app. She took down her stories but she put that one for sell on amazon.
The story is "Until You" by Kristina Dalton