This was back in 2013. At that point I have been developing games (as a hobby and then professionally) for 13 years. During my free time in high school and college, that's basically all I did. And then right out of college I got a job in the game industry. I also developed and published my own game. In 2011 I moved to the bay area, joined a game startup, and it was acquired. (By Zynga, so not that exciting, but overall, I'd say things were going well.)
Around that time, I realized that there was no way making games would help with x-risk. So I left the industry. This involved letting go of the deepest passion and the most developed skill I've had at the time. It involved changing which circles I networked in. And it included abandoning all the knowledge I accumulated of the programming libraries and framework, game design, game lore, and all the half-baked game ideas I had and was hoping to develop some day.
I still stand by that decision. But I've also found a way to incorporate a bit of that old self into my present life. For example, sometimes I design a board game. Or write down and explore game ideas until I can "see" how it would be developed. Or dream about having enough money to just hire an entire studio and have them develop it.
Magic.
I'm a magician. I let go of magic because [1] I realized that I'd rather be a scientist or engineer than an entertainer, [2] I don't like talking about magic with laymen and [3] it's hard to give others your full attention when in the back of your mind you're figuring out how to deceive them.
I almost never perform magic tricks anymore. Fortunately the autodidactic skills I learned from magic transferred well to everything else in my life, as does the sense of presence I learned to project.
A few that come to mind:
1. A 6 year time investment/lots of emotional investment in a relationship, once we realized I wanted kids and adventure, she wanted no kids and stability.
2. 2.5 year time investment in a startup, as well as ~$50,000 of others money, as well as a lot of emotional and identity investment, after our deals fell through and we realized that there wasn't enough demand to sustain us without those deals.
5. A 5 year time investment in a small business, after I realized I couldn't have the impact I wanted working on it.
I suspect a big one for many will be a religion where sunk costs can be in the thousands of hours
Most culinary preferences of my youth, since marrying.