So far, I am quite happy with where I am in life, even though nobody ever goes through their younger years without making mistakes.
I would probably tell myself to have a lot more focus and discipline. It takes discipline to bed early every night, and my sleeping habits were terrible. Unfortunately, it did somewhat have a negative effect on my academic performance, even though I still did very well, but I'm a perfectionist so I always believe I could've done better.
I would also tell myself to have more patience and focus on self-improvement, and never stop after accomplishing a target because there's always a next step.
Read more books and learn more about the world, workout regularly, and ditch those friends you had that didn't share the same mindset.
This list might be updated in the next 5 to 10 years. I may be a perfectionist, but nobody is perfect, and I will continue to make mistakes down the road, but learning from the those mistakes is what matters.
And you? If you could time-travel, what words of wisdom would you give the younger version of yourself?