Awe shucks
Seriously though if you cant believe in yourself and your abilities then it becomes helluva difficult to convince others.
Just do it if your wrong so what everyone makes mistakes - jeeze I remember I made one in 1984

Two peiieces of advice that sticks in my head are
1) As a teenager and my first day on the market stall, the old stall holder beside me said "son you better have some better ideas for selling than the cheapest price, put a bucket of shite on there for £1 and there will be 5 in this market selling it at 50p next week" and you know what he was right - it taught me to stay away from the mainstream and seek out new opportunities and market them for as long as I could, this still holds true today even more so in the world of e-commerce selling cheap devalues you as well as your products.
2) I was once told to understand the difference between "need" and "want" which I apply to my business if i need something I buy it, if I only want it I question and try to justify the purchase.