All writers have to get used to rejection. It's a rare writer who has never been rejected. You need to don a thick skin in the morning, especially before the mail arrives (snail or e).
You won't be surprised to learn that many subsequently famous authors received their fair share of 'thanks, but no thanks' notes. Here are a few:
Crash (1973 by J.G. Ballard - 'The author of this book is beyond psychiatric help.'
A Study in Scarlet (1887) by Arthur Conan Doyle, introducing Sherlock Holmes to the world - 'Neither long enough for a serial nor short enough for a single story.'
The Last of the Plainsmen (1908) by Zane Grey - 'I do not see anything in this to convince me you can write either narrative or fiction.'
The Blessing Way (1970) by Tony Hillerman - 'If you insist on rewriting this, get rid of all that Indian stuff.'
The Rainbow (1915) by D.H. Lawrence - 'It is unpublishable as it stands because of its flagrant love passages.'
Peyton Place (1955) by Grace Metalious - 'Definitely too racy for us.'
Animal Farm (1945) by George Orwell - 'It is impossible to sell animal stories in the U.S.A.'
Leaves of Grass (1855) by Walt Whitman - 'We deem it injudicious to commit ourselves.'
and two of my favourites:
The Time Machine (1895) by H.G. Wells - 'It is not interesting enough for the general reader and not thorough enough for the scientific reader.'
The War of the Worlds (1898) by H.G. Wells - 'An endless nightmare. I do not believe it would take... I think the verdict would be "Oh don't read this horrid book".'
However, if you do get rejected, I do recommend you don't respond with any invective or abuse, such as our alien wannabe scribe: