Why Did I Not Believe In Myself

I'm an engineering student and was searching for internships. I found my dream internship that asked for everything I had to offer and was the only one I really saw myself enjoying. When I finished my application I just couldn't bring myself to press 'Send' because I kept thinking that my application wasn't as good as it could have been. Then the day came where the offer was taken off as applications had finished. If only I had believed in myself and sent it, now there's nothing I can do.