Easy one.

I would tell myself to get a degree in a field that gave me a good job and not in a field that would "change the world" . I now know that I have much more fun with my hobbies than I ever could on anything serious and that I will never "change the world" with my career choices. Life would be easier for me now and far less stressful and more productive if I had a "good job" not training for a dream.

I was to much of an idealist, and not enough of a scholar.