|The filling makes all the difference|
The middle of your book/story is really important. If you went to a movie and they only showed the first third and the last third all you would do is get introduced to the characters and the problem and then see how they resolve everything. You'd miss out on the building intrigue, the budding romance, the impending doom, etc. that makes a book or movie so much fun. A book without a well written middle is like a sandwich without anything in-between the two slices of bread, blech.
One example of the importance of "Act 2" in a book is Sisters Red by Jackson Pearce. Sisters Red is about two sisters, the older one incredibly disfigured from saving the younger from a werewolf, and the older sister's werewolf hunting partner. I just finished reading Sisters Red and really enjoyed it. Why? Because I got to watch the evolving relationships of the characters so vital to the story that without them the ending would have had far less impact on me. And the majority of that important stuff happened in the middle of the book.
So as you are writing anything fiction, whether it is a short story, a novella or a full-on novel, be patient with yourself as you embark on Part 2 of your work in progress and remember that it is probably the most important part. If you stick with it and work your way through, using the finale as your reward for perseverance, you'll likely have a great story that will draw people into the world you've created. And that's what it's all about, isn't it?