Back in the day, I'd start writing out a fantastic idea, get some of the way through it (usually not even halfway) and then abandon it for lack of motivation. Eventually, I realized what was wrong: the source of my motivation.
Before, I began new stories frequently because they seemed like interesting ideas I'd enjoy. Then as soon as the novelty wore off, so did my motivation. It was no longer fun because it was no longer new. I'd give up on it.
After realizing this, I sat down with myself and retooled where my motivation sprang from. I set a goal. It was simple: FINISH THE DAMN STORY.
I told myself: If I have the brain capacity to start a story, I certainly have the capacity to finish it.
Finishing might be hard, but it's doable.
So I buckled down and got to work. I encountered snags and plot problems, pulling at my hair when I couldn't think of what was supposed to happen next. But each time I got held up, I persisted. I often took breaks, but did not give up. I found ways to work through the problems. I kept my one goal in mind - to inch toward to that finish line. With each push, I got closer and closer, until one day... I FINISHED. It wasn't brilliant and will never win any prizes as is, but by golly it was finished.
For me, it was a matter of telling myself what I wanted. Was I controlled by whims? Or goals? Whims flutter about, and your writing becomes a leaf cast about by the wind. Goals are a solid point that you aim for. No matter where you might wander in your writing, if you keep steering yourself back toward that goal, you will eventually reach it.
What if I'm sick of my story?
It's a sign you've been spending too much time on it. Set it aside. If you've got it in you, work on a different writing project. If you're feeling burned out, take a break from writing altogether. Play video games. Read books. Watch movies. Do fun things to rejuvenate your tired brain. Then, when the joy has returned, get back to writing.
What if I can't fix my story?
Not all stories are meant to be finished. There will be times when you start something, and you keep pushing and pushing simply because you don't want all that previous effort to go to waste. Remember this: you have no obligation to finish it. (Unless you've got a contract with a publisher, in which case, congratulations! And also, sorry, now get your butt back to work.) If you're at a point where you're unsure if you should trunk the story or not, have someone else look at it. An objective set of fresh eyes might be what you need to regain motivation to continue, or give you the nudge you need to put it down and start something different.
* Feeling motivated? Maybe click on that Vote button for me? :)