Check out this wonderful post by Zoe Winters at Indie Reader.com
Too often we feel the need to compare ourselves to the success of others. It's really an unhealthy thing to do because there is no gain in it. If someone is doing better than you all comparing will do is make you feel like crap. If someone is boasting about better sales and higher royalties than you are currently getting, it's going to make you start to question yourself as a writer.
Am I good enough?
Are my books crap?
Why am I not selling as well?
These questions are self-destructive. As a writer/author, your only ambition should be the love of writing and sharing your stories. Yes we all know money needs to be made (we all have to eat), but constantly second guessing yourself is only going to prevent you from writing more. While your first story may not sell well, future work might and if you're spending too much time worrying, you won't be writing.
Zoe says it so much better than I so please stop in and have a read.
You'll be glad you did.