There are certain things that I don't like:
1) Tying my shoes
2) Making a grocery list
3) Ending a story
It's hard to decide when to end a story, and maybe even emotional if you've grown to love and adore each and every character, villain or hero. Here are some tips to help you decide where to cut off the story. Keep in mind that most of these won't apply to series stories.
Tip #1: TIE UP ANY LOOSE ENDS
You don't want to have your readers with any questions except for: Why didn't I find this book earlier? If they have any questions after the story is over, it's not a good sign. It means you weren't clear enough in your explanation of the questions that you made for the characters to think about. Find a way that will fit through the story that you can explain what's going on. Have a character reflect over everything that's happened.
Tip #2: MAKE THE LAST CHAPTER WORTH IT
Make up for what you lack throughout the story. Didn't have enough romance? Have the characters engage in romantic actions. Didn't have enough drama or action? Make a fight scene as the post-finale. Comedy was a little low? Sarcastic puns never hurt anyone. You don't want to focus too much of your attention in one category, spread around so the story will be varied.
Tip #3: CLOSURE IS KEY
Do not- I repeat- DO NOT leave your readers on an eternal cliffhanger unless you plan to make a sequel. You can shove as many cliffhangers as you want for the chapters before the last because they'll be fulfilled. In Lycan, I left my silverlings with a cliffhanger that was continued in the next book in the series, Luna. However if you leave a cliffhanger at the end of your novella, you'll be making the biggest mistake of your writing life. The readers need a sense of completion and closure after reading the story you wrote, otherwise it won't be truly called a story.
With these tips, I hope you'll be able to end a story with no regrets and feel confident about how you ended it. I really want every single one of you to succeed in your writing, it's an amazing journey going through a book and bonding with all of the characters that you brought to life. It's such an emotional experience that you'll never forget :)
Writing is truly a creative art - putting word to a blank piece of paper and ending up with a full-fledged story rife with character and plot. - William Shatner