I read a lot of fiction submissions for Clarkesworld Magazine. Several submissions a day, so on a normal week I read about twenty to forty stories of varying quality, and I've been averaging about that many per week for most of the last year and a half. Before reopening to submissions on January 15th of this year, CW had a word count limit of 1,000 to 4,000 words. Since reopening, we've been accepting up to 8,000 words per submission.
When we had the smaller word count limit, many writers I met at conventions told me that they would love to submit stories to CW, but they couldn't write stories that short. Now that the limit is higher, we get more submissions, but in my opinion the longer ones could nearly always use some serious editing for length. It's absolutely fine for people to submit stories up to 8,000 words long, but in most cases the stories I've seen that are that long could be written much more memorably without so many words.
Here are the main problems I see in stories that seem too long:
1. Too much exposition. Backstory explained at great length.
2. Too much description of surroundings. Unnecessary passages about the appearance of things that aren't significant in the story.
3. Simple wordiness.
The Exposition Problem
First of all, it's important to look at your reasons for including information in a story. What's the core of the story? What changes for the protagonist, and why? Exposition is often an attempt to show what is normal for a character, but it can go too far. It goes too far when the telling of a short story starts with things that happen days before the inciting incident that gets your character to act. It goes too far when it explains too much of an alien society, or the political structure of your fantasy world. If your story relies upon a lot of this kind of description to make sense, then you may have a novel on your hands; it may not work in a short format. But maybe you can find a way to tell the story you want to tell without including so many details about the whole world where it takes place.
Choosing the most important things to show will strengthen your story. You need to reveal what your character wants by showing her in action. Stay close to her efforts to get what she wants, or to avoid what she's trying to avoid. You might (and probably should) know everything in her background that leads up to what she wants, but you don't need to tell the reader all of it.
For good examples of stories that convey a lot in only a few pages, you can read "A Letter from the Clearys" (Connie Willis), "The Way Down the Hill" (Tim Powers), or Clarkesworld's Nebula-nominated stories like "Non-Zero Probabilities" (N.K. Jemisin) and "Spar" (Kij Johnson). I'd also recommend reading the shorter stories in any "Year's Best" anthology in the genre of your choice (up to, say, 15 pages long). Study how they convey information. It will help you to learn how to express ideas efficiently.
Description Could Kill Your Story
The reader needs to know where and when a story is taking place, or at least to be given enough clues to form an opinion, but going on and on about the setting slows a story down. The beautiful castle may sparkle with gems, the scent of roses wafting upon the air, with strains of fairy music uplifting the spirit in every room; the walls may taste of candy when one licks them! But if the story isn't moving forward, even the thought of characters licking the walls won't save it.
I'm not saying not to describe the setting at all. I'm just saying you should judge long passages of description with your objective pants on. Does the reader need to sit through all of that? Are you describing something of particular importance to the story? Or are you being a little self-indulgent if you leave that part in?
There's a Shorter Way to Say It
Once you have your draft finished, and you have a clear idea about what the story means to you and what you want to convey (you've thought about that, haven't you?), then it's time to really scrape away the excess. I have two masters to offer you on this subject. You've heard of Ernest Hemingway? You can get plenty of advice from that guy without leaving the internet:
Ernest Hemingway’s Top 5 Tips For Writing Well (Brian Clark)
27 Secrets To Writing Like Hemingway (Joanna Young)
More importantly, I'd like to get all writers to read The 10% Solution (Ken Rand). In this little book, there are practical, concrete steps listed that show you exactly how to reduce the length of your manuscript by 10% (at least; I've taken out up to 12% of my word count in some of my stories by following his advice). You can retain and even strengthen the meaning of what you wrote by looking through your work mindfully, examining the types of words and phrases you've chosen and thinking of tighter ways to put things. You use your own opinions and end up with your own voice in the work.
But Don't Skimp on the Ending
Nearly always, if a story is too long, the problem is in the beginning, the middle, or both. If a story is too short, the problem is in the ending. One of my top reasons for rejecting stories, back in the 4,000-word-limit days, was abrupt endings. It was as if the authors had looked at their word count, panicked, and chopped off the last page or two. A story needs some resolution, a bit of closure about what's changed and a sense of what might happen next. Even an open-ended story shouldn't let the reader fall off a cliff and go splat.
I suggest another approach. Instead of cutting from the ending, start the story later in the timeline. Copy the draft into a new file, and remove the first few pages (start with three). Look at how you could start the story there, and sprinkle backstory lightly into the middle, or leave it out altogether. Why not try this, just once?
The whole point of the short story is to create a strong impression with few words. It takes confidence to leave unnecessary explanations out, and to believe in the reader's ability to co-create a story with you. Confidence always helps.