I majored in writing (fiction track) in college, but none of my published work has been fiction -- mostly because getting fiction published is an extraordinarily arduous process, and you don't typically get paid for short stories unless you're already an established author, so I haven't written much since college. I'll try to offer what advice I can though. Please excuse the formatting: I'm at work at the moment, so it's easier to add bullet points as I get time than trying to write a long, cohesive response at once.
The biggest mistake I see from most amateur writers is trying to combine drafting and revision, or foregoing revision entirely. These are very different processes, and however you might feel about the quality of your first drafts, both are absolutely essential. I think it was Bonni Goldberg in Room to Write who first referred to drafting as "making clay": you can't craft a pot without generating raw material first -- and on the other hand, without revision, all you've got is a roughly pot-shaped lump of clay. The process I've found works best for me is to sit down and write as much content as I can without pause, then go back and chop out most of it and proceed to gradually refine. For every three pages I write, I might end up keeping one, but the more raw content I produce, the more selective I'm able to be.
One of my professors (who's well-known for her flash fiction work, where, even more than in short stories, every word needs to count) used to have us do an exercise in class where we'd write a sentence and then spend a good ten minutes re-writing it in as many ways as we could find. I strongly recommend this exercise: the more you do this and observe the subtle (or not so subtle) differences in tone the various word and structural choices produce, the more intuitive it becomes.
Experiment with different perspectives. Sometimes a story that isn't working in third person does in first, or vice versa. Or if it isn't working in first from one character's perspective, maybe you're reporting from inside the wrong character. It's also important to keep in mind that there's no such thing as an objective narrator: even from a detached third-person omniscient perspective, you're always making choices as to which details and events to show and which remain "off-camera", so try be aware of that and know why you're making the choices you are.
In terms of structure, every story falls into one of two categories: "a hero goes on a journey" or "a stranger comes to town". In the case of the former, the story begins when the protagonist sets off to fulfill some desire, and in the case of the latter, it begins when external factors disturb the order. In either case, the protagonist will typically strive eventually to return to where he started, having learned or grown in some way from the experience. Similarly, an adage among playwrites describe the three acts of any play (which also apply to television, film, and most fiction): get your character up a tree; throws rock at him; get him back down. If you're ever struggling to find your story, framing it in terms of this basic narrative structure should help you find some direction.