I you could tell the 20-year-old version of yourself anything, what would it be?
If, by the magic of time travel, I could get a message to myself, it would be to get rid of the ego. Seriously, put it on the shelf and leave it there until you are much older and have actually accomplished something.
The most a 20-year-old can have is talent. It takes years to turn that talent into something and I wish I had approached everything without ego. I did have a great deal of talent at 20, but I was not the greatest thing since sliced bread, despite what I believed. I wish I had approached my life with confidence and humility. Humility allows learning and opens doors that cockiness never will.