Rejections, Rejections, and More Rejections!

Okay, folks! I am curious to know how you deal with rejections. How many is too many? Should you ever give up? Do you ever have those days, after receiving so many rejections, that you say, "I must be a horrible writer if no one wants to publish me," and you just want to throw in the towel? Let me hear your thoughts on this. What's the best/worst rejection letter you ever received?