There's been a lot of talk flying around lately about writers needing a thicker skin. I, for one, truly believe that you need a thick skin (a few layers even) if you're ego is going to survive being a writer.
I'm not just speaking of handling the rejections from agents, editors or publishers either. Even before you get to that point, you need to prepare to take some harsh criticisms of your work, unless of course, you believe you are one of those people who can write without the need of any editing. For those that believe that, you need much more than just a thick skin.
When you hand off what you consider your finished manuscript for others to read or critique, be prepared to hear some unflattering remarks. I know it hurts. I've been there. There are few rules I think you should follow at this point.
One: Do not email, phone or get in touch with your reviewer - not yet. Take the time needed to rant, rave, spaz out or whatever you need to do, and make sure you do this in private. Once you have taken your moment, or day, go back and reread the comments again.
Two: After your sanity has returned, try to incorporate some of the suggested remarks into your work. As hard as it to believe, many times you will find it works better. This can be hard to swallow, so make sure you have a glass of water by your side.
Three: After your rant is done and your manuscript is looking better than ever, take the time to email or call your reviewer and thank them for their time and suggestions. Even if you did not use a single suggestion to improve your work, you still owe your reviewer a thank you. They have taken the time to read your baby and their comments were meant to help it be the best it could be. What they don't need, is your whining, hostility or any arrogant defense of your work.
Do that, and you'll never have to worry about that kind of criticism again; no one will ever offer to read over your work or help you out anymore.