I’ve only been married once (now for 32 years), but I had two long-term relationships in college, both of which lasted for about two years.
The first one was probably doomed from the start, due to the age difference, if nothing else: I was 18 when we started dating, and she was 32. We had wildly different life experiences: I was a goofy college kid, while she had been in the Air Force, and married and divorced twice (with two kids) by the time we had met. It was also a physically-driven relationship, and she eventually got a wandering eye, and dumped me for another guy (who was also younger than she was). We stayed cordial, but were not terribly close after we broke up; I haven’t heard from her, or heard anything about her, in at least the past decade.
The second one was with a woman who was much closer in age to me: when we first started dating, I was 20, and she was 18. It was a great relationship, we had a lot in common, and we were starting to talk about marriage. She then, quite suddenly, broke it off; her stated rationale, at that time, was that she was pre-med, and planning to go to medical school – due to that, she felt that she would have to devote most of the next decade of her life to that as her sole focus, and didn’t feel that she could be the partner and spouse which I deserved. What she didn’t vocalize back then, but which became clear within a few years after our breakup, was that she was more attracted to women than to men. The two of us have always remained friends, and there’s still an emotional (but now platonic) closeness that we share; ten years ago, I attended her wedding, and her wife was thrilled to get to meet the guy she’d heard about for years.