How to get over disappointment about an idea?

I had thought of a very interesting idea for a book that I fell in love with. To make sure it hasn't been done, I typed the keywords into Google...and to my displeasure, the idea is already a book. I'm really saddened by the fact that I can't do this idea now - lost interest since it's been done. How do I get over it? And has this happened to you? :/