I believe it is fair to say that every adult has regrets, and can point out several moments in their life of which they would say if I had known then what I know now, I would have done things differently. Such feelings are part of human nature, and I guess it is the knowledge of how we failed in the past that makes us better capable of dealing with similar situations later in life.
But still, many people fantasize of how things would turn out with all their hindsight knowledge available to them. My upcoming novel Revenant Rising partially deals with this issue as well.
Now let’s suppose such a thing were possible. Suppose there was a way for you to travel back into your earlier self and still retain all the knowledge you have accumulated since (regardless of how far you travel back). For this mental exercise we’re going to assume the trip is one-way, to rule out nasty stuff such as time paradoxes.
Your first challenge is to adjust to your new surroundings. Suppose I were to travel back to my 17-year-old self, I would quickly need to refamiliarize myself with all the things I just knew, but have forgotten over the years (for instance: which houses required which newspapers on my paper round, and which courses at school were doing whatt subject at that time).
Your second challenge is to keep the people around you from freaking out. Telling anyone you have knowledge about the future will at best give you frowns and at worst land you in a nuthouse. Also, your mannerisms, your accent, your interests, all of these might have changed in the years since. If these deviate from your normal behavior back then, then you might give yourself away.
Now, supposing you have done all this, and you have adjusted to your new surroundings, and are now ready to do things differently with your vastly increased knowledge, how do you proceed? Even the smallest change can have drastic effects on how things turn out, and your life, despite whatever flaws you perceive it may have, may turn out worse.
But let’s assume that is not the case, and your change is so small that it has no major repercussions on your future. You are now stuck in the past in your old self, and need to live your life again from that earlier point. How will you do this then? Do you make the same decisions, leading back to the same life, or do you change other things as you go along? The former might be a rather dull life as you’ve already lived through everything once (getting to experience the good parts again is of course fantastic, but living through the bad parts again might not be all that pleasant). But the latter has its own problems. First of all, all your knowledge of the future will become more and more obsolete as you change things. Sure, many major events will probably still happen the way they did before, but even so your small meddling might influence them. But the further you deviate from your own history, the more you will influence those around you, and how their lives turn out.
The eventual question becomes: how much can you endure? At first your knowledge will make you feel powerful, omnipotent even. The world is at your feet, and you know which mistakes to avoid. But as time progresses, you will notice the differences between your old and new life. Even the smallest change could trigger a chain of events that prevents you from meeting the man or woman you loved in your old life, or prevent another person you care about from meeting the love of their life. Your life is connected to so many others, that every step you take can influence those of others. Every step you take now has the potential to destroy something that would otherwise have happened, and all the while you will have perfect knowledge of how things would have turned out otherwise. How long can you endure this, knowing what you have destroyed in your selfish quest to change your own mistakes? Regretting your own mistakes is one thing, regretting the things you unwittingly destroyed quite another.