Oh this is an easy pick, even though there are two major contenders. The runner up is me not taking the pain in my arms seriously, and as such, ending up with both arms semi-fucked and in intermittent pain for 1.5 years and going, which is seriously problematic since it hampers general computer usage, which includes gaming, game development and writing.
My pick, however, has caused me an awful lot more distress than that. My biggest regret, bar none, is that i didn't change schools when i was a kid. As such, i ended up enduring 6 years of bullying, and it broke me. It's been over 8 years since i left that place, and i'm still dealing with the effects. I know full and well how silly anxiety may seem to an outside observer, but good fucking lord, out of all the things i have to deal with in this existence, this anxiety is the worst, and I COULD HAVE AVOIDED IT. Do you know what it's like to have a burning passion in your heart and a head full of ideas that you can't act on because of a subconscious fear that you have no control of, and can't do anything about? It is dreadful beyond words. It's like bearing your own goddamn instruments of torture around. All the things you see from me are basically only the tip of the iceberg, poking out from the dark depths of an ocean of fear. And i could have avoided this. So there's a regret for ya.