I have discussed dealing with the rejection letters before. After you send out your query letter you face one of the most frustrating parts of the submission process, the waiting. I read an interview with Stephen King once that said he would submit a short story or novel and then fill his wait time by working on another project immediately.
The most difficult part for me when I first went through the submission process was the frustration of being rejected over and over. I would convince myself of the quality and desirability of my manuscript but then not only would I be told it wasn't going to be accepted, I was turned down in a mass produced form letter with no personalization whatsoever. Knowing that those I submitted to most likely had decided without even taking the time to read the story I had worked so hard to create.
When it comes to traditional publishing it is a necessary evil that must be dealt with no matter how difficult it can be. I got very discouraged at first when I was starting to receive a steady flow of the rejection letters but it helped me to know Gone With The Wind was rejected 38 times before it was published. William Faulkner was told his story was unpublishable and Gertrude Stein submitted poetry for 22 years before she was published.
The biggest piece of advice I think we can all agree on is that as painful as it can be to get rejected, it is worth it to remain determined and keep pushing through. For the letters that are more personal, see if there is anything you can learn from them. Did the agent or editor make any suggestions? Could you send a query letter or first chapter to a friend or trusted beta reader to make sure that you haven't missed anything? We have already talked about making sure you edit your work before submitting but by enlisting the help of different people you may get different pieces of advice or have things stand out to one person that someone else may not have noticed.
Stephen King's decision to keep his mind occupied by working constantly is something I have found inspirational. I have two currently finished novels that are going through editing and will have two more completed by the end of the year so once I have the current ones ready and begin submitting I can get the next two through editing and then I have outlines and research ready to go for another half dozen projects. We all get rejected it is just a matter of being persistent enough to push past for that elusive acceptance that makes it all worth it.