> But obviously in hindsight, it was the most expensive mistake I've made in my life.

Maybe a tiresome pedantic response, but this is only a "mistake" to the same extent that it was a mistake for [everyone in the world who had the required funds] not to buy 100 BTC in 2015. If you can overcome the cognitive biases (endowment effect, loss aversion, etc.), the fact that you previously owned the 100 BTC has no real bearing on the situation, beyond transaction fees and a couple hours max saved by holding (doing nothing) vs. buying.