I’m not going to sugar-coat this. Endings are ,by far, the most frustrating parts of every story. For most beginner writers, their stories tend to end unrealistically.
Keep in mind that not all stories have to tie up many loose ends as you may want to set up for a second book. They just have to end in a satisfying, believable way that doesn’t end to quickly or drag on for too long.
When writers end their stories too quickly it’s usually because they feel that they have worked on a story for too long and just want to get it over and done with. In these cases, the endings tend to happen very suddenly which doesn’t sit well with a lot of readers. One of the “kiss of death” reactions to any story is if at the end the reader thinks “Oh…that was the end?”. The ending to a story needs build up to that climatic moment page by page and absolutely cannot be rushed. So when writing an ending, take your time. Just think and work things out. You’re almost at the end and you don’t need to race.
Another problem that faces writers while writing their endings is the “too long” ending. While endings shouldn’t be too quick, they shouldn’t drag on too long either. After you’ve written your climactic moment, don’t try to keep the story’s action climbing. At this point, wind down the action and bring the story to a close. For some writers, they find they want to keep writing because they’ve grown a fondness to their characters. They’ve come to know them better than anyone, were there every step of their journey, through highs and lows. Therefore saying goodbye is too difficult for them. They don’t want to end the story because they had so much fun with their characters. Although it’s difficult, a writer needs to know that when it’s time to wind down and let go of a story. Just remember it can’t last forever.
All I can really say is while you’re writing your ending, make sure you work at it. A good way to avoid either of these bad endings is to try to plan out your ending before you even start writing the beginning so that you can build up your characters and plot throughout duration of the story in order to bring your story to a structured, believable close.