Good question... Somehow if we can get out of the hyper rationalist maximization seeking robot mentality, an answer of some form may present itself. Accepting death helps. Not just my own but that of much more. Buddhism has some very interesting insights here. And I bring up belief deliberately, because the answer here is faith. Not certainty, but faith. Whatever it may be (leave legacy, do no harm, etc), it's doing what you believe that results in a satisfactory answer.

If you were born into a very tough situation in a war torn or third world country, you'd do what you needed to with what was given to you. While we do have a lot of material wealth and might have more free time, we still face the same great equalizer: death. As do our civilizations and our planet (and likely us, way before that). Given the constraints, you can simply "try your best". Living without regrets might be a job to one person, a 90 hour startup to another, traveling and learning to others. Whatever your belief is, following it is the best that you could do. Ultimately you have to answer to yourself & be able to live with yourself. Personally, living selfishly, while it maximizes certain things simply goes against my core beliefs. I don't necessarily know that living less selfishly is necessarily better. But for me, it is. Your beliefs, may defy logic, and may not necessarily maximize legacy, wealth or whatever, but in having acted you will have done your best given the situation to be able to live with yourself, which is all you can ask of yourself.