The woulda, coulda, shoulda that lives in the back of our minds, born out of regret. Many of us stumble through life wondering about our past; what would have happened if I had done this, should I have done that, etc. And the worst, having these thoughts on our death-beds. Well, the past is a moot point; it's gone, over, finished. We cannot go back and fix any perceived wrong that we think we've created. It's done. But, we need to know that what we have done in the past does have its effects within our present. Understanding this gives us a leg up, and that is a big deal. Also, it should teach us that what we do in the present weaves our future - a point that should not be forgotten.
It does not matter what we do; something or nothing has its effects in our lives. Yes, even non-action, a pacifistic way of life (not wanting to get involved, going with the flow, let somebody else do it, living like a drone, etc.), generates its effects; think about it. History, both global and personal are full of examples. So, monitoring our behavior should be an important thing for us to do. But don't stop loving, don't stop giving, don't stop living a life of virtue; don't live a life hiding in a corner waiting to die.
Just don't live a life full of regret. If you want to climb a mountain, do it. If you want to find your true love, go for it. If you want to jump out of an airplane with a parachute, happy landings. If you want to find Moby Dick, knock yourself out. If you want to pedal your bicycle around the world, do it, just be sure to carry extra tires!
When it comes to doing things, it does not matter how old you are - make a list and work to make it happen. When you are at the point of passing from this world into the next, do so without regret and the anguish of not knowing. Do so with the thoughts of, yeah, I did that and so much more.