"I am stuck. I've been working on this story for months now and I just can't get anywhere with it. I thought it was because it was a story that wasn't ready to be told or shouldn't be told but I realized last night that I'm having a hard time with it because the whole premise feels like I'm stealing someone else's idea. It's not fan fiction but the idea did grow from a love of someone else's work and I keep feeling like these characters aren't mine, even though they're not from the original work, and that I shouldn't keep writing their story. I'm not a real writer, so does it matter?"First off...if you're writing something, whether it's for public consumption or just for yourself, you're a "real writer." There's no special test to take in order to become a real writer; there's no income requirement. If you're putting words to paper (virtual or otherwise) and you do it because to not write feels like you're not paying attention to an important part of yourself, you're a real writer.
Secondly...there's nothing wrong with getting ideas and inspiration from something that already exists. It happens all the time, often becoming wildly popular...you know, from "real writers."
Let's set aside the 50 Shades books; everyone who hasn't been living in a cave probably knows those bubbled out of the frothy brew of Twilight fan fiction.
Ever watched Sons of Anarchy? Awesome show, wasn't it? Would it surprise you to learn it was a retelling of Hamlet?
How about Diana Gabaldon's Outlander series? I read recently (no proof of its truth, but I love that it might be) that the inspiration for that was Doctor Who.
Look how many times Romeo and Juliet has been retold.
Write what you love to write. If it's fan fiction, go for it (but don't try to publish it without permission of the copyright holder.) If it's a retelling using unique characters, go for it. If you're inspired by a TV show or movie or another book, tell that story the best way you know how. Make those characters who are whispering in your ear spring to life, and give them a wonderful, ultra-high-def, colorful existence.
Don't ever rip off someone else's story using your own characters, but it's fine to draw inspiration from everything around you. That's kind of the way it works much of the time.
We're all stories in the end. Just make it a good one, eh?