It's not a question of "long-distance relationships" being hard, of themselves. They are, but to know if it will work, you have to look at all the other variables. Chinese have been doing long-term, long-distance separations for many decades, probably centuries, and somehow it works for them, in its own way.
They work when there is a specific, goal-oriented reason for the separation, such as making a living or one or both parties pursuing education. It's best when the separation has a specific beginning and ending point.
In other words, they succeed when the separation is necessary and/or geared toward advancing the family. "Once I finish this PhD in Alabama, I'll get a good job and we can buy a house." Obviously, this suggests that the partners already have a high commitment level, such as being married or at least engaged.
Your statements that "China seems like a fascinating place" and that there is "lots to learn" and that you can "meet all sorts of people" are good reasons to go. But they aren't going to sustain the relationship. And, more to the point, if the relationship had any future, you wouldn't be thinking of leaving in the first place -- not for these reasons, anyway. That's why I say the relationship has run its course. Going to China or not going won't make any difference.
Somewhat subconsciously, China is probably your escape route.