In another fandom that I'm in, there were a few writers so prolific that they update daily with each chapter amount to 4-6k words on average, the story is posted while the author is still writing and I can never understood that level of productivity.
(It's insane, no? Agree with me!)
Writing has always been fun, but very hard for me. I'm only following one story at the moment. In the past, I am always flooded with plot bunnies it was ridiculous. It is easy to feel tempted to jump to new ideas and abandon the old ones, leading to finishing absolutely nothing in the long run. Right now though, I just had a huge note file where I keep the ideas for the story I'm writing, and laid them down before making an outline. Whenever I came up with something cool (that back then would have turned into another story completely), I tried to cooperate it into my current story if I could. It also helps when you know what you are writing before hand.
It also helps a lot to have a sounding board, someone who listened to your ideas and give feedback before you write your story. Other than that, it's just pure crushing through until it's done for me.