Well, first of all, it may be partly that short stories aren't your thing. If not, that's all right. But even so, I can understand wanting to learn to write them.
I'm also inclined to keep expanding on my ideas until they grow to be monstrous. I'll come up with subplots, give my minor characters backstories, you name it. But personally, I'm not bothered by the idea that a story I write, whether it's a novel or a short story, might only be part of the "whole story" I have in my head. I know I develop my stories and characters more than I "need" to, but I don't mind.
I've written short stories about characters from a novel of mine that didn't work out. I did it by isolating parts of those characters' stories that could stand on their own, and developing them.
I definitely recommend reading more short stories to get a sense of how they're structured. But also, it might help to imagine how those stories might be different if they were novels, or parts of novels. Don't know if that would help you or not, but I find it helps me put the difference in perspective.
Also, when I write with a specific word count in mind, I do try to hone in on an idea that I feel I can make work well in a story of that length. If you find that you can't imagine scaling an idea down so that it'll fit in a short story, it might be best not to force it.