I am a bit lucky to have some product skills to identify the mental biases and traps that I fall into, and some friends help pull me back too. Other commenters have pointed out some of those problems:

- Look for positive signal from real flesh-and-blood customers who will give you real money. I printed out my content and had lunch with former colleagues to pick their brains on the topic. I kept doing small tests to check one single area of the final concept.

- Downscale. Downscale. Downscale. You might conjure up 100s of reasons why you can't dare approach the market with a faint shadow of your final vision but you need to do it to test that you aren't making a huge mistake. They will not love it. You will not make money. But you also will not waste time doubling down on a fantasy. Gumroad is good for this but I'm doing it on Shopify just by reducing my product to 10% of the content and printing 1% of the order quantity I need to make a real profit (so I will lose a few hundred, but I will validate if I should spend a few thousand)

- Figure out how to market. You want to do this because this is a form of your "sales funnel" and how you will actually identify who you want to attract, how to do it, where to go, etc. I'm highly unskilled in this topic but I'm using a book called "The 1‑Page Marketing Plan" which helped me structure a lot of things that I had observed in life and start to build a proper plan in steps that was logical and connected. It's not the best book ever, but it is good enough to get oriented and do something logical.