Many writers make the mistake of thinking that great writing is something that simply happens, and that writing skill is something you either have or you don't. If you don't "have it", you're doomed to obscurity and failure for the rest of your life. However, nothing could be further from the truth. It's actually something that's cultivated through time, practice, hard work, and yes - even paying attention to constructive criticism when you're lucky enough to get it. Rejection slips are simply part of the process of the long journey to publication, and learning to deal with criticism and rejection are necessary lessons on the road to becoming the writer you really want to be.
More often than not, rejection slips come right along with reasons why the writing in question was rejected - sometimes very detailed ones. Instead of seeing the rejection slip, and anything the editor has to say about WHY your writing was rejected, as a personal attack that you're entitled to stew, weep, and pout over, you should be trying to see it for what it is instead - absolute gold. It's valuable advice from someone in the business that knows what they're talking about, knows what sells, and knows what makes compelling writing - something a lot of writers would kill to get. Read their comments over carefully, consider what is said, and look for ways you can apply what you learned to your writing so that it can be better. You can't produce truly great writing until you identify your bad habits and figure out how they're affecting your craft. We all have them, but only people who know how to value constructive criticism will get past them.
Rejections also help you become stronger and more determined over time, not only as a writer, but as a person. Sure that first one might really sting. Probably so will the second and the third, but after a while, you learn that rejection and criticism are simply things that come with the territory. Even the best writers in the world can paper a wall with the number of rejection slips they got before they finally got to the point where they were selling their work. Some received an absolutely ludicrous number that would make MOST people give up before finally tasting sweet success, so look at your rejection slips as a rite of passage - something that you have in common with the greats you look up to. If you learn to put them in proper perspective, each one gets you that much closer to the day when you get to squeal over an acceptance letter instead.