First off: on the word-count/page-count thing... a page in manuscript format is usually about 200-250 words depending on how much dialogue is in it. You're looking at 30+ pages once you get it ready to send out.
Yeah, you can sell things at that length, but the shorter the better for most markets. I second Gray Rose, though. That's not WHY you want to cut, that's just a happy side effect.
Short stories need to have even tighter prose than novels. (Ideally, novels will have extremely tight prose as well, but there is more breathing room in the longer form.) Yes, magazines want to give really good stories first - but here's the thing. They also want to sell copies.

If you can say "Contains stories by: BIG LIST OF AUTHORS HERE" you're more likely to hook readers on one or more of the authors. Magazines will have a maximum page count, so there's only so much space per issue. Long stories take up more space, which is why in most markets they have to be 7500 really good words.
Recommendations:
1) Let the story marinate in its own juices for a little bit. Let yourself work on something else for a while and get a little distance. Forget what you wrote.
2) Go in with a sharpened blade and cut. The 10% is a guideline, you might have to cut more. If you can't find a purpose for a line, cut it. If you have two sentences that do the same, or almost the same thing, cut one of them. If you have two sentences that do two separate things, cut one and revise the other to take up both roles.
3) Once you think that the story is ready - find a critiquer. If you don't have one already, Share Your Work is a good place to get some extra eyes on your work.
4) Take all of your critiques and then decide which ones to follow. Using your critiques effectively is another skill that you learn by practice.
The thing about cutting - don't feel discouraged that we're telling you to lose large chunks of the story without seeing it. I have read a lot of 6000-10000 word first drafts. Even the very best of the best of those could stand to lose upwards of 1000 words. It's just a function of the first draft process.