I have decided Novellas are the hardest thing in the world to write. I have written some and they are usually fairly successful but I find it takes me as long to write them as it does a full length novel. A short story is fairly easy to decide what to write because a short story from one to five thousand words relates an incident that has an impact on a person. There is little room for a lot of description and backstory. The emotion and the lesson to be learned is where the story shines. The characters are ones who have a real problem. One of mine is about a doctor who was sued for malpractice when he stopped at the scene of an accident and while he didn't lose the case, he lost his practice. This story starts when he comes across another accident and it involves his decision to stop or not stop.
A novella has more word but not as many as a novel. Therefore the author has to decide what to include and what to leave out. I find I often leave out things that should be put into the story. In a novel I would have no problem deciding what to include and what to leave out. This is an area where I really can't make a decision when I'm writing short. I usually end up with what I think is wonderful but confuses my critique partners and other readers. I may never write one again. I can write real short or long but the medium length totally baffles me. If anyone has any ideas about how to accomplish this, let me know.