Relax, it isn't as serious as it sounds.
Sometimes, as a writer, you have too much on you're plate. You've mentally committed yourself to various projects. Or, maybe the deadline for the particular story you're writing is looming, but the story just isn't working out. You have no idea how you'll meet the goal.
Unless you're contractually obligated, it's okay to give up sometimes. It's better to let a story fall to the wayside than to stress out over a story that will turn out to be crap and not be accepted anyway. The old saying "quality over quantity" really applies here. You want everything you put out to be the best it can possibly be, and that's hard to do when you're concerned about getting the thing done.
Don't feel bad if you have to give up. Just because you don't finish the story now doesn't mean you'll never finish it. There have been many times when I have to put a story on hold because I can't think of what to do next. Then, months or even years later, I'll suddenly have an epiphany, and I will finish the story.
Of course, before you give up on your story completely, you should try to see if there's some way to tweak it. A few weeks ago, I was writing a story for an anthology of true ghost stories. I was writing about my first encounter with a spirit (long story), but it wasn't long enough to reach the 4,000 lower word limit. I was about to give up on it until I decided to add an experience from college. I was able to finish the story, and it got accepted.
Giving up is part of the business of being a writer. You have to know when to stop working on something bad and when to keep working on something good. Hopefully, you'll have more "keep working on something good" moments. Those are the best.