Hmm. I used to feel like that. Honestly, though, I haven't written many short stories—but, strangely, my take on them has evolved. I feel a lot more comfortable with the idea of writing them than I used to. I guess I'm more used to writing for a certain amount of words than before (or at least, I feel like I am—and that certainly helps a lot).
Anyway, I might recommend making a fully contained plot that could also be a single chapter in a novel, or a side-story from one. Short story plots aren't always as full-featured as novel plots, anyway.
I think one thing that helped me in my feelings was to listen to more fairy tales, and short stories—including juvenile fiction. Listening to juvenile fiction is nice, since it helps to expand the horizons more such that one doesn't feel necessarily restricted all the ins and outs of adult fiction. More things can happen in juvenile fiction, and they don't always have to make as much sense. Similarly, I don't think short stories have to, either—but it's nice if they do once in a while, anyway.
I'd recommend reading The Book of Dragons, by E. Nesbit, and maybe some of the fairy books that Andrew Lang compiled.
I guess I felt more restricted before I got into reading / listening to short stories. You say you read a lot, so maybe you're already used to reading them—but, maybe not. I know I didn't like the idea of reading short stories until recently (mostly because I hadn't read many that captured my interest).
Also, I'd recommend feeling free to put some humor in the short stories. One thing I liked about The Book of Dragons was the humor in how unrealistically fast the dragons seemed to be at eating large things, and how little they satiated them. With things like that, I don't think all the novel-type features are as important, but then, probably, it can alter your audience severely.
