Unrealistic writing goals

Everytime I set out to write a really really long novel I never make it past 20k. Everytime I plan out a massive fanfic for a show I adore I never make it past 20k. Nor a single draft in my Google docs has made it beyond 20k it's like a curse. When I check the word count and see 20,000 I can feel the story just stop working. 

Completely unrelated but for some reason it's never occured to me to write a 20,000 word story.