A few years ago, I became disgusted with life and publishing, for reasons that I've already adequately enumerated. A writer doesn't stop writing, of course, so I wound up writing fanfic-- close to two million words of it. Here's what I've learned from writing fanfic:
1. How to write short. When I started writing romances, long, long ago, my natural writing length was about 100,000 words. I eventually learned to write novellas of 20,000 words or so, but it wasn't till I started writing fanfic that I discovered I could write very, very short pieces. I've written short stories with satisfying endings as short as 400 words. Of course, fanfic comes with its universe and characters already established, so you don't have to do setup. It's unlikely I could produce a satisfying original story of 400 words, but I can now turn out decent original shorts (with sex scenes, even) of 5000 words or so.
2. How to write angst. As a romance author, my published stories all had happy endings. That didn't really fit with my mood after my husband died, and during all my health problems. So I indulged myself by turning out the occasional piece of angst, with endings that were really, really sad. And to my surprise, I can do it darn well. I don't know if that is a skill I'll ever be able to use in professional writing, but it's nice to know I can write tear jerkers.
3 How to write differently. I've tried all sorts of different stuff-- first person, present tense, dialogue only, stories told through emails, slash, and even a "literary" type story or two. These are all things I probably wouldn't have done professionally (except the first person, perhaps) and I think they all helped me develop some new skills and learn more about writing.
4. How to have fun. Most professional writers know that the endless grind of searching for agents and publishers can have a really bad effect on the ego. Fanfic has nothing to do with trying to get published, and there's not a lot of stress involved; it's just writing what you enjoy. People generally don't read you unless they like your style, so you typically get mostly positive feedback. That's fun too:-).
5. How to write every day. I've spent the past several years writing nearly every day (I've slacked off some this past year due to health issues). It's easier to write for feedback. But it also helps to have a set time every day just to sit down and write, and to make writing a priority in your life.
6. How to use Photoshop. Well, Photoshop Elements, actually, and clearly I don't know how to do anything fancy with it. But I've spent a couple of years making my own title pages for my fanfics, and I enjoy playing around with images as a break from writing. At least I can make a banner now, which is something I couldn't have done before.
So fanfic has been fun. I think, however, that if I can translate some of these lessons over to my professional writing, it might begin to be fun again, too. We'll see!