I've been writing a lot of short-stories lately, and I slush for a magazine that publishes short-stories. Although you said you wrote hundreds of flash fiction pieces, so you're clearly a more experienced writer than I am (I'm still in the dozens), I'll try to convey some knowledge anyway.
I tend to frown on short-stories that (mostly because of the author's lack of experience) read like "mini novels". They have massive worldbuildings. They start with a made-up quote telling how the world was created by incestuous gods. Then, the story cuts to an orphan kid in a farm. Don't do that. These people just don't get short-stories, they're trying to fit a novel within 7,500 words.
That put, write a story with a well-determined time period, preferably one single event in the live of the main character. It doesn't have to be a major event, as many succesful short-stories are about mundane, day-to-day things; just don't write about the MC's whole life, from birth till death by old age. Of course, the event may lead to his death, or something worse..
The ending doesn't have to end all things. It will will only end that single event. And the story doesn't have to be closed-ended. The best stories have ambiguous endings that remain in the writer's mind for years. I believe that it was Argentinian author Cortázar that said that you write a short-story in three steps: 1) write a story with beggining, middle, and end; 2) cut the beggining and the end; 3) whatever's left is your short-story.
You should start with a strong opening line. I'd advise you to start right in the middle of the action (or
in media res) and don't explain it. Just let it happen and the writer will get what's going on, i. e., don't pull a flashback to explain how the characters got that, it will ruin everything. Apex Magazine's editor-in-chief Jason Sizemore wrote
a neat blog post on openings.
The ending must be as Aristotle said: unexpected and inevitable. Unexpected because the reader must fell from his chair (figuratively or not). Inevitable because the reader will scream, "Of course!". Think about Poe's
The Black Cat. To write an ending like that, the development must lead to it. So, every scene in the middle must take the story more towards its ending. If it doesn't, you should remove it.
Notwithstanding, watch this guest lecture by Mary Robinette Kowal:
Now that you're out of excuses, go write. (totally not stolen from a well-known podcast).