Listen to mamamia — she gives many pearls of wisdom in her post.
But don’t give up writing!!! You have two options:
If you’re determined to work on THAT story, you need to contact your friend and get her agreement that you can finish it without her. Get it in writing. If she’s OK with it, finish it.
But If you either don’t want to push it to the point of getting it in writing, or if she refuses to release her rights to the story, then for Heaven’s sake — write something else!!!! If the thought of starting whole new book from scratch is too intimidating at first, then start with small steps: Write some short stories; write just some quick scenes; do some of the prompts here; start a journal; write a list of ideas; invent some characters; anything! Just put some words onto paper, and you’ll get momentum going.
But write! Wanting to put thoughts onto the page is a gift — use your gift!
Best of luck and good writing!