The main problem that I have is knowing when to end it. A few months ago, I was typing up the last chapter of a fic and I just couldn't end the scene. It kept going on and on. Finally, I just had to close the window.
That's really my problem when it comes to writing. I just want to keep adding to the story. Like the one I'm writing now, there's a two-year time skip, and I want to write all the details of what happens in those two years. I have to force the muse to move on from that.
Then there's the fact that I can get distracted unless I write at a certain time or don't do research as I'm writing. But this creates another problem in that I get inspiration to write, but it's at a time where I'd be really distracted by something.