For most authors, receiving a rejection letter is about as common as that bag of potato chips in the pantry; you never eat, nor do you receive, just one.
For me, the first 'many years' of my writing career were spent sending out query and cover letters only to get a typed form letter back addressed, "Dear Author," as if saying "dear" somehow excused the publisher from knowing my name (even though I had to know theirs :)
I could have saved all of my rejection letters like a friend of mine did, and tape them together to use in future teaching opportunities, (rolling it out across the room at the appropriate time) but I was so upset after receiving them, the idea never occurred to me.
Today, I don't write any more queries or cover letters, but I can tell you that the experience will never be forgotten.
What I want you to know is that receiving them isn't the end of your life, whether you receive the form kind, the kind with your name written on the top, the kind that are personally written to you, or even the kind that say, "We love your book, but have concerns about this and this. Re-write and send the book back." Even the kind that come to you with a hand written note on the form letter are something to write home about.
But in the end, you are still receiving a rejection letter of one form or another, and should take a look at the concerns (if any are mentioned) and do your best to get your manuscript out to the next publisher.
I used to have a numbered list. When the manuscript came back, I would check it over, make adjustments as necessary, and send it right off. And I tried to have a few different manuscripts out at the same time so that if one came back I could tell myself, "Well, at least I haven't heard from so and so yet," and that would help me to hang on.
Rejection letters are, simply put, someone else's opinion of your work. They may like it; they may not. What's most important is that you don't give up on selling or publishing your work because someone you may not even know, has given you a thumb's down.
You wouldn't want to quit when your thumb's up may be just around the corner.