You can't, so don't bother thinking about it. You can make changes today that will pay dividends in 10 years, so maybe focus on that instead. I was widowed pretty young (as widowers go), so I have done my share of playing the what-if game with the past. Ultimately it is not a productive exercise, and I try not to do it. If I catch myself dwelling in unlived lives, I try to refocus on the one still in front of me.
I tell any young adult who will listen the mistakes I made. It's nice when they stop by with stories and whiskey the days before Christmas and see them come up strong, happy, and on a solid path towards success. It's only through the stories of our mistakes that we can escape the clutches of time.
"Vicarious learning from the experiences of others saves making errors yourself, but I regard the study of successes as being basically more important than the studyof failures. As I will several times say, there are so many ways of being wrong and so few of being right, studying successes is more efficient, and furthermore when your turn comes you will know how to succeed rather than how to fail!"
~ The art of doing science and engineering