I have lots and lots of failures, so I write an email every week with one of them (https://softwareclown.com/).
Biggest one... negotiate for your salary? Start looking for job after first round of layoffs? Don't deploy into production with new language in way that can break biggest customer's service?
Nothing's more important than your self's well-being.
What marks my biggest failure as my biggest failure is that there is absolutely nothing to be learned from it.