Paraphrasing Rand, I think I would tell my younger self: Do not recognize anyone's right to one minute of your life, nor to any part of your energy, nor to any creation of yours, no matter how large their number nor how great their need.

Personally, I think I spent too much time in a futile effort to please parents and relatives and so forth. Later in life, I find that is not only a futile effort, it can also make one completely miserable, since one is not being one's truest self, but is acting as one perceives others want them to act or be. Reading Rand, I find that mistake is a common but grave mistake, underlying societal decay in much of history, namely, the mistake of living for others instead of oneself. So yeah, that would basically be it. If I could reach back a decade, and tell a copy of myself in a Everett-Wheeler parallel universe one thing, that would be it, the above paraphrase from 'The Fountainhead'. :-)