I guess my biggest regret is never letting my original instructor know how much I really appreciated what he did for me before he passed. If I could turn back the clock, I'd love to have a long talk with him about it.
Now that I'm older I realize meting out pain or damage is the easiest part of training martial arts. Lots of people can do that. A lot fewer people know the other side.
I was stationed on Okinawa and I worked with Angi Uezu. I could have spent my free time learning Isshin-Ryu where it began, instead of drinking at the club every night. Wish I had.
There's a couple things I wish i had done when I was younger (like starting earlier), but I really had no control over them, so I don't look back with regret. My parents didn't have an informed opinion about Martial Arts, and only let me sign up in the first place because all my closest friends were already going, and they thought it would be a fun after school activity for me. It just so happened that it was Tang Soo Do and an outstanding school with a great instructor. My parents actually regret not letting me sign up sooner more than I regret not being able to do so.
I also wish I had more time to keep up regular training during my time in the Marine Corps. But, for all 7 years I was either in a war or training for it, so I had no control over that either. Still, even taking all the bad things with it, I wouldn't trade those experiences for having kept up with regular MA training instead. I'm more than happy being a 2nd Dan trying to get back into the swing of things, instead of a 4th Dan who didn't answer the call.