I once wrote about how i like marathons because success or failure is unambiguous-- you either run the time you wanted to or you don't. Life is not like that, but on the other hand it is foolish to evaluate it aspect by aspect as if life were some sort of checklist to be completed. I'm not even sure if there is such a thing as success or failure in life, other than the basic requirement of continuing to exist. About all you can do is reflect on the choices you've made and contemplate if where you've arrived is very far from where you might have been.
I turned 50 about a week ago, and nice round numbers cause one to reflect. It's futile to compare your life at 50 to childhood aspirations, or at least it is depressing unless maybe you are Elon Musk. When i was in high school, i thought i would be a billionaire and also win the Nobel prize, so i was kind of set up for disappointment. At that time i didn't even think of the things that have made my life good, like my sons, or the peculiar satisfaction i got from running long distances. You also don't imagine the ways that life will damage you if you live long enough: the lost friends, the opportunities that can't return, the injuries both physical and psychic.
I struck upon a test of happiness several years ago that works for me. It's not a metric per se-- you can't use it to compare your happiness to someone else's, provided that you both pass the test. The test (which i've probably stated before) is this: would you trade your life, in full, with another person? Not just facets of that person's life, so you can't trade for his or her career success, or his or her appearance, etc. but everything. Would you give up your friends for that person's friends, your family for theirs, etc. If not, then it means that you have something in your life that you would not trade, and that is a sufficient criterion for happiness to my mind.
I feel fortunate that i have people and activities in my life that i would not trade for anything. But this is also an age where one is young enough to enjoy what life offers but old enough to have a fairly clear idea what those things are. Music can still make me downright joyful, and good food and wine can ameliorate a lot of stress or disappointment. Even with my replacement parts and diminishing abilities, i enjoy the ineffable feeling of the good days running on trails or biking up the Coast highway. The older i get the more i appreciate those things that are just there, the things that don't need my effort or responsibility or care.
There's a line attributed to John Barrymore something like "a man is not old until his regrets replace his dreams". That's the approach i'm trying to convince myself to take. My failures and regrets at 50 are fairly numerous, but not burdensome. Even another five years, or ten, could see some dreams fulfilled or maybe some new dreams formed. My mantra is essentially: improve and explore.