Most confidence advice comes from people who have never lost anything.
I'm not one of them. I've had the wins and I've had the disappointments, more than once, and I kept going.
I sold on Amazon using FBA, Fulfilment by Amazon, for 9 years. I wasn't selling my own brand. I sold about anything I could source and I built the business up slowly, along with relationships with my suppliers.
Then a bot flagged my account for something and asked me for information I couldn't provide.
So I asked what else I could do to fix it. The answer was always the same: "Give us X." I didn't have X. That set off more restrictions, and every one of them sent me back to the same loop. Give us X or you can't sell. I don't have X, what else can I do? Easy, give us X.
Nine years of work, building gone and I was stuck with thousands in inventory that I couldn't sell and couldn't return. For a while I felt like I couldn't recover, embarrassed when people would ask "Hows the business". Got down on myself for not being able to "Fix" the problem, letting my wife down.
After that, I stopped trusting myself for a while. If you've ever thought "I don't believe in myself anymore" or "I feel like a fraud", I know that feeling from the inside.
Then I realized that it wasn't personal, just me, many many people had similar shutdowns. I could use what I had learned over the years in the business and apply variations to LOTS of different businesses. That loss didn't define me, it was just a BIG learning experience. Get up, dust of and keep searching for something new!
One thing I still use is simple. Write down what actually contradicts the belief that you're a failure, then pick one manageable challenge and do it today.
This site is for people getting up after a business that didn't work, a deal that fell through, or a bet on themselves that didn't pay. It's confidence built after the knock, not before it.