The other day, a friend of mine complained that he was only able to write 200 words of fiction at a time. After that, he was hitting a block he could not pass.
Here's my response:
Celebrate those 200 words! If that's the burst you write in, so be it. Make it part of your process. Write 200 words, then get up and give your family hugs or do 20 sit-ups. Then come back to the keyboard and write 200 more. Repeat.
I don't know if that helped him, but it's how I work (not that I write much fiction anymore, but it still applies).