Nail on the head. this true love stuff is best left in under funded TV movies. Happens all the time. Circumstances can force one's hand in a relationship, any relationship in a good or bad way. Beyond basic incompatiblity if a couple are pretty good together(no one's perfect, if some think that's not true they're daft) and have been together long enough t know each other and have a loving relationship, it's amazing how easily outside circumstances can force them apart.
One of the two women I truly loved in my life is a good example. We loved, respected and cared for each other very much. We still care for each other now although she's in love and living with a great guy for the past 5 years. Anyway, enough back story. When we were together, she got an amazing offer to work and live in another country and I couldn't have gone with her at the time. There was no way around it that would have realistically worked and we looked for one. We were young and this was the best and probably only opportunity for her to really live her life and follow her dream(which worked out for her too). We split up. In fact looking back, I dumped her as I knew she wouldn't have gone. We were both heartbroken about it, but I couldn't have lived with myself if she had stayed for me out of love. Now we were a good couple, objectively as good as or better than 90% of the couples I've known. We were compatible in all the ways that count, yet we split up due to outside circumstances. Hey maybe some sap may say if it was "true love" it would have worked out or some nonsense. I consider what we did and how we did it truer love than you find is often defined as such.
That's not that unusual an example. I've known others where for whatever reasons, even reasons from within the relationship that could have been worked on, but for one reason or another it didn't work out. Some of these were great relationships. Sometimes, it's not a plan, there's no pattern, the universe doesn't get involved and as inlove123 says sh*t just happens.
BTW, I pretty much agree with the first part of your post and I wouldn't worry about your age. If you're getting this stuff pretty straight in your head at 18, you're ahead of the posse by a looong way.