Advice from an impatient reader: if it doesn't move the story forward, or it's not needed for character development or scene-setting, leave it out. Elmore Leonard says it takes him 4 pages of rough draft to come up with one finished page - not that we all should write like Elmore Leonard, but his books do move along at a nice clip.
I didn't get far enough into the story to encounter the backstory others mentioned. Backstory is tricky. Sometimes you can work it in as you go, rather than dropping it all in one big load.
The grammar of this sentence caught my eye. I'm no english major but it looks like the tense changes.
I stood up and dusted the sand off my jeans, walking up and grabbing his halter before the horse could bolt as the man obliviously opened the gate and walked through it.
I'm sure an editor would have caught that, but a better solution might have been to tighten up the sentence, leaving out unnecessary details (like the sand on her jeans.) It would also create more tension if the man opened the gate first, causing a momentary crisis for the protag to solve. This would also justify her pissy mood, which was a turn off for me. (It's not a guy thing - pissy guys annoy me just as much as pissy women.
Without an editor, I think you, or anyone for that matter, needs more than a quick once over before posting a first draft as a finished draft. At least give it a few days and then come back and take a fresh look. Also, budget enough time to fix things during your "once over". The urge to post a fresh story can sometimes make us impatient and unwilling to be objective about our work.
Nitpicking, I know, but that's what you asked for.
(I'm on my sixth "once-over" for the contest story I was planning on submitting two days ago.