The key thing I have my authors keep in mind when they get overwhelmed or frustrated is that publishing is a journey (sometimes a long and winding one), that I am here to help them, and that everything is a stepping stone to the end goal. If you get passes from editors, their feedback is the stepping stone to revising your manuscript and getting into better shape. If your editor sends you a horrible cover, it opens up a conversation about the elements you don't like so that we can get the cover to be something you're happy with. If you're not getting the publicity you were hoping for from your publisher, you at least know what they will be doing for you and you can plan how you are going to fill in the holes and supplement their work.
Being an agent means, in part, that you help your author see the big picture. Having worked with hundreds of manuscripts and been a part of many deals, I know what is normal and what is a disaster. For instance, I recently had an author freak out that everyone in her first submission round passed on her project, but when I told her I've had authors who have gone through dozens of submissions rounds, it helped her put things in perspective and stay positive. I use the stepping stone speech a lot, and find that it really helps authors calm down and think about the bigger picture.
Also, for #IHAVEAQUESTION, thanks, Diane, for being the first to ask! She wanted to know, "How do you approach a publisher about a book? Is this a casual lunch or a bustling happy hour?"
There are a myriad of ways to approach publishers about books! You (and by you, I mean agents) can do it while networking at events or when out to lunch or drinks. For me, though, the most common way I talk to editors about books is via email. I write a pitch similar to what authors write when submitting to agents, and tell them about the project or, if I know them really well, write a quick note about the book and say that I think they would love it. The editors I email are people I already have a relationship with 80% of the time, although it is not unusual for agents to research an editor's interest and email them out of the blue!