It is impossible to know, with certainty, if any decision is the best one because we only live one life. We can make educated guesses about our "what if's" but we'll never know. And furthermore, you have to accept that the road not taken will also have disadvantages and trade-offs that you could not have anticipated at the time.
There are some decisions that you can be pretty sure are big mistakes. And some of these are irreversible. And we are all running out of time.
But generally speaking, I tend to agree with the Proustian perspective -- the "big moments," generally speaking, are the product of a lot of little moments and decisions. Spouses don't suddenly decide to cheat one day, careers aren't made by one or two wrong moves. Rather, I think we only remember the moments in which all of our little decisions culminate into one event -- precisely because it is memorable.
You have to learn to love your fate. There are so many branching permutations of quantum lives that it's impossible to ever know if you chose the "best" one.