Sometimes you need to limit yourself and don't over think it. Slam down an outline. Start, plot points, ending. Fill the blanks with some sense. Then stick to that as close as possible while you fill between the lines. Only change the outline if you read the draft and it's a clusterfuck of nonsense.
It can be a good thing to:
1. Just write. Let the story tell itself. Your strongest characters will define themselves a lot of the time.
(Accidentally hit reply)
2.Switch between projects. Sometimes smaller stories you can't incorporate into anything will clutter your mind.
3.As for being motivated, there's no real trick, sometimes you're just not. Go do other things you like and write down story ideas, and play with them in your head till you get to a point where you can write.
Speaking from my experience, I usually come up with ideas first, then make an outline for the story, and then just kinda let my mind turn the outline into the story. At I wasn't able to fill the blanks between the point in the outline with much, and the points that I'd expand weren't too great either, but as I got more and more practice, these segments started to get better too.
Of course, sometimes you're in no shape to write - that can happen, and that's fine. You should beat yourself up about it - that'll only make you feel worse and make it more likely for you to be able to write the next day.
What helps me write?
The fact that I can't draw.
But the written word is no match for a good drawing hand. Visuals are more enticing than text. I'd would gladly trade my writing skills for those of a professional draftsman.