I had my first serious (and only) relationship at 19, and ended up marrying him. Which is great in many ways, although I sometimes wonder if I have 'missed out' on anything by not dating around more.
I started dating at 20, married her at 23, and nearly 24 years later we're still together and have three teenagers (yay...).
I would agree with what others have said, that you have not missed out on anything that you would be better off for having experienced. Though of course I'm in the same boat as you and you might fairly consider my perspective to be lacking in this area.
If there's something that you have to do often, it makes sense to learn to cope with failure. If you are doing maths homework and you can't get a proof to come out right, it's good to examine that and develop strategies for the next time you get stuck, and also to make sure you keep trying and don't just give up. But that's because you know you're going to get more maths homework, and you want to do better next time.
But with relationships, if you get it right the first time, you don't have to go and do another one the next week. Instead you can devote your time and energy to the one you have already established and nurture it, which is better for all involved.
Would life have been different if you'd chosen someone else? Yes, and undoubtedly it would be better in some ways and worse in others. But that's not the point. It's not about trying to find your "soulmate" (a poisonous notion for which popular media has much to answer). It's about making a choice and a committment. Part of that committment is not spending serious time wondering about the other choices you could have made. The occasional "what if" isn't a problem, but if you find yourself asking it more and more often, you need to start paying attention to the choice you did make, because there's work that needs to be done. Any successful long term relationship needs continuing work and investment, just as a home or a car needs maintenance. It's easy to let it drift until the minor, unnoticed issues suddenly build up to something big, and a ceiling falls down or steam starts coming from the engine. We all have to guard against that tendency.
As for whether adversity makes you stronger - really commit to a marriage and over the course of a few decades you'll undoubtedly encounter all the adversity you need. It's not in short supply! There's no need for nor moral benefit to piling it up ahead of time.