> it's not that hard
Yes, that's the greatest component of my regret. I cannot even make the excuse of "it was so hard, and I was focusing on other things".
I started weightlifting at age 40. I never seriously exercised or did sports regularly before. After one year of exercising 30 minutes a day, I am in a better physical shape than ever before. Those 30 minutes a day are totally worth it. And I regret not doing it twenty-five years ago, when I had much more free time than today.
Why did I even think it would be hard?
Seems to me that most people are bad at giving "80/20" advice -- an advice that is not literally best in the world, but is close enough to the best, and it easy to understand and remember. People often make things sound much more complicated than they are; for example there are dozen of exercise methods, and everyone says their own method is the best, and doing any other method is just a stupid way to hurt yourself. It all you have is contradictory advice of this type, it doesn't sound encouraging. Then some people say "well, it's a science, you have to study it", but I don't have extra 5 years of life to research exercise, then 5 more years to research nutrition, 5 more years to research finance, etc. for each important aspect of life.
(Then there is another line of useless advice, which is "just do what feels natural". Well, what feels natural for me is using internet all day long, and not exercise. Because that's what I have been doing most of my life. And of course, my natural shape is fat. But this is all because "feels natural" simply means "your habits". If you have good habits, following them is a good advice. But if you have bad habits, it is a harmful advice.)
If I tried to give advice to younger people, I would probably just give them one book per topic to read. "Convict conditioning" for exercise. "How not to die" for nutrition. "Early retirement extreme" for financial planning. I would have to think longer for books on other topics; perhaps some topics do not have the right kind of book. (The right kind = one you can read without much previous knowledge in the area; and which will give you advice that is maybe not perfect, but pretty good and not actively harmful.) I believe that sending these three books by a time machine to a 15 years old me would have dramatically improved my life.