Running out of money. We were bootstrapped, and the style of work we were doing had some decent capital costs. We failed for a "bargain" (less than 7K), but we had cut to the bone and deeper while trying that. Side jobs to make rent just increased stress and distracted from the business.
Why did we run out of money?
Focused on engineering instead of sales and marketing. Manually spinning up servers would've been the right thing to do--since we instead were both technical we solved technical problems and kinda missed the people side of things. It was real and hard work, but it was not work that was relevant to making sales.
After we ran out of money, why didn't we keep at it?
Stress blocked further work. Too much stress led to neither founder being able to even think about the company and its business without severe anxiety. Times were dark and bad.
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I learned that you want to prioritize the money funnel over literally every other thing that you as an engineer think matters. Coffee is for closers.
I learned to try out only with other people's money. Don't put your rent and basic living on the line for an idea if you aren't comfortable with what that can entail.
Probably most importantly, I learned that the glorification of the startup lifestyle is a lie perpetuated by people seeking to make money from it at all levels. Nowadays, even the glorification of failure and celebration of mental health seems to be by people looking to advertise their businesses, cure-alls, or just get internet famous.
It's rotten to its bloody core, and unlike finance doesn't even pay well.