While I was in grad school a professor of mine told the difference between a successful writer and an unsuccessful one, and that difference is a large stack of rejection notices. The writers who do make it have a tough skin calloused enough to shrug off little pinpricks of momentary setbacks. The unsuccessful ones stop sending out their work after a dozen rejections; they go on to become accountants for writers who do make it.
All you can really do is keep going. If you can't, then you're going to have a problem. No book, no seminar, no yoga technique will ever be able to help you as a writer if you don't develop early on the ability to take rejection like a man. Form letters should be ignored altogether; the only purpose they serve is to notify you that one magazine does not want a particular story. Personalized letters are infinitely more helpful as they may clue you in to a specific flaw that your work might have. And acceptance letters...well, if you feel bad about getting those, then you really need to have your head checked.
There is, of course, a limit on how long a story can continue pounding the pavement before it's time to retire. Here's my little formula: send a story to five or six publications. If they all reject it, sit back and give it another revision. Most magazines take about a month or two to response to a manuscript, and that time away from a story gives you the distance to go back to it with fresh eyes as though you're reading it for the first time; problems you overlooked earlier might stand out more clearly. Revise the story once or twice and then send it out again. If it gets rejected another five or six times, then it's probably a sign to retire the story and start a new one.
I would also ignore the rule about simultaneous submissions. A simultaneous submissions is exactly what it sounds like. It's one piece of writing sent out to multiple publications at the same time. Most magazines don't like this. Editors, I hear, are slow to forget this, and I think that's more of a business move than anything else. They don't want to accept a story and get geared up to print it only to find that a competitor made a deal for it. However, as a freelance writer, you're job is not to ensure that one magazine stands above all the others. Your job is to get your story in print sooner rather than later.
As an example, Asimov's Science Fiction responds to manuscripts in about five weeks, and they're one of the quicker magazines. Some take up to eight months. That's a long time to wait for a single yes-or-no answer. In this fashion, a short story can take a couple of years before it's either accepted or you decide to retire it. Folks, that just ain't right.