Steep?

String Theory and related math. Math is not my strongest suit, so taking the same courses a decade earlier would have been much easier and more productive.

Some musical education. I have no relative pitch to speak of (or at least a gross mismatch between identification and reproduction), but early training would have at least enabled me to carry a tune without being horribly off key 100% of the time. It sucks that I can't even sing to myself without cringing.