I guess I would tell myself to stay in college and to become a lawyer. Then I could have made a good income and probably married a fellow lawyer (I'd give myself some advice about choosing a spouse on more than infatuation/sex appeal) and had a few kids while I was young enough to chase little ones around the house. We would have had enough income that I still could have had a horse and travel.
But, I'm so glad for the experiences I had instead, even if it took me 16 years to get a bachelor degree because I kept dropping out to do things like move out of state with a boyfriend, live abroad, etc.
And although I regret not having perpetuated my genes, I'm glad I have the daughter I adopted in my mid-40s.
No other horse could have been a perfect and lovable as the horse I had. Oh, and the feral cat I captured and brought home from abroad would have had a short hard life instead of a long spoiled one.