When you have found stability in a relationship with a woman--which really means having committed to her--your present condition changes the color of your past relationships with other women.
All of them suddenly become mistakes, some happy ones but for the most part unfortunate errors of judgment, either yours or by the past beloved. No matter how in your daydreams you curiously wonder how your life may have fared out if you ended up with someone else; no matter how in your weakest moments you even desire those lost possibilities--your past history of love must for all intents and purposes become a history of errors.
As time reinforces your love for who you are with; as custom and habit gradually plant you to an inert state preventing you from moving back and forth; or more importantly, as you delve deeper into the person--the "real" person who can never be seen from the distance of courtship that separates those who have not committed to each other--as you get to know her more and more, she transforms right before your eyes: from a wild bet into the future or a leap into the unknown, she becomes a certainty, a truth.
From being one of the many, she becomes "the only one." All the rest, those you had desired, those you had perhaps loved even more, they will now pale in comparison to her--and necessarily so. And this not because of petty rationalizations or "hindsight" or absurd self-consolation, as if a person can suddenly turn from being a mistake into correct decision! The one you have committed to becomes perfect for you because you realize she was the only one you fought for and stayed the course with so as to discover more about her through time.
You do not sacrifice your youth and say a little prayer and give a chance for every young love to grow. Or at least, you shouldn't. Do not stake everything for every woman who parades in front of you. Your youth is the only thing which you shall never reclaim (you'll have a lot of time, perhaps too much time, for old age). This is the sticking point: your past loves compared to the one you have committed to were, after all, not worth the risk when the time for a thundering decision came--either love was not there at all, or you had already seen soon enough what will become of your future together, that it will only be the same at best, or it can only go downhill from there. You did not bet on or fight for every woman. But you did for this woman. The truth of love is won in both senses of winning the prize and fighting for victory.