I regret not descending faster on dirt roads in New Mexico, on dirt and gravel roads everywhere I've ridden. You work so hard to even get to a place and then you work so hard on the bike to get to the top of that place and then waste the downhill with worry.
Because, you know you're not going to get to the bottom, turn around, and ride back up to descend it better.
Wait a minute. Maybe what I really regret is not turning around to descend it better. Either way, I'm not making that mistake again.