Ahh this is such a biggie for me too, almost as much as having zero interest in my stories. XD Have you tried writing in a genre so astronomically different from your preferred one? If you write fantasy, try writing a contemporary. Or if you write romance, try sci-fi from a 10-year-old’s perspective. Maybe you write a wide variety of fiction. Try your hand at a non-fiction. This can prompt a whole new learning curve – and be lots of fun too. 😛 Or maybe it’s time to take a break from learning about writing and start learning about the other aspects of being an author – stuff like marketing and query letters and the pros and cons of self-publishing vs. traditional publishing.
Or you could test your betas and write something as badly as you possibly can, then hand it over and see what they have to say about it – if they just fix the polishing things or go after plot and stuff as well. 🙂 Hope this helps!