I would likely start off by just smacking me around for a while, but afterward, I would talk about priorities.
I would talk about what really matters, and what really doesn't. I would talk about marriage, and what it means to be a great husband, how it will feel a lot like hard work, but a spouse that you love unconditionally becomes your closest friend. I would talk about being a Dad, and what it takes to be consistent, how to love your kids how they need to be loved, not just what comes easy to you. I would talk about how you will never regret any effort you put into supporting your kids, but you will regret every opportunity you missed. I would talk about friendship, how the friends that selflessly want to see you grow are hard to find, so give back to them, and lean into them when you need to. I would talk about how no one was meant to be an island, and great friends make the most difficult of seasons much more bearable.
How much heartache I would save myself and others if I could go back and speak those things to the younger me.
Unfortunately, that opportunity is never going to come to me, or to you.
But don't just get frustrated, there is something great that you still can do.
Proverbs 13:20 says "Whoever walks with the wise becomes wise, but the companion of fools suffer harm."

God has given all of us a measure of wisdom. Through mistakes and victories, we learn things that help us to live more effectively. So share that information with the younger generation God has placed around you. Save them from some of the heartache you experienced. Save them from making the choices like the ones you made that hurt others.
In addition, God has given a measure of wisdom to those who are further down the road of life than you, listen to them, learn from them. After all, sticking solely to what you know, and only spending time with those who are in the same place as you, will cause you to "suffer harm."