I genuinely believed that I wasn't intelligent or confident enough to pass a Google interview. When a Google recruiter contacted me several years ago, my wife encouraged me to reply even though I liked the startup for which I was working and didn't think I'd make it. I prepped aggressively, got very lucky, and now I've been at Google for over 3 years.

This is a truly wonderful place to work; if you are interested in being here, I'd encourage you to apply in spite of your self doubt. :)