First startup went nowhere and took on a contract role after 9 months. Then tried another startup with a co-founder I met at one of the startups where I worked in the interim. About 6 months building something awesome, but no commercial path. Spent 1 month with him and another co-founder on a fintech product but realized much faster that once again that there was no viable go-to-market strategy. Started another company and built a product that seemed like it had legs. We had one early user that absolutely loved the product and we thought all we needed was to find more users like her. Turns out that she was a false signal because we never found another user like her and I'm about to shut that one down after almost a year to avoid DE franchise fee next year.
If anyone is in a situation like this/thinking about doing something like this, I've gathered some of my lessons learned: https://chrlschn.dev/blog/2024/12/lessons-learned-from-worki...
If I had to sum it up: always build the minimal thing that can be "sold". Use AI to build the dirtiest MVP as fast as possible. Even better if your "MVP" is a deck and you can get people to put money down to wait. Figure out your GTM and messaging with that deck. If you are an engineer, you must resist that urge to build until you're sure you can find enough people that have this problem and want to pay you to solve it. Don't work with a non-technical co-founder if they claim if you build it, customers will come. Don't work with a non-technical co-founder that can't demonstrate an ability to sell. If the vibes feel off, get out fast. Don't form a company with a co-founder until you absolutely have to (like your personal life, don't get married until you're absolutely sure).
Lots of mistakes and lessons learned during that time having founded startups that went nowhere, been an employee in startups that went nowhere, and left startups that are actually crushing it. I have seen a big swath of the gamut at this point with some regrets in retrospect.