For those who are going to watch this movie for the first time and to those who have watched but are still confused of the ending, watch the prologue because it is the main explanation of the epilogue itself.
Also, I think the main problem on the female side was his money. She never wanted to take advantage of him (-as she said at one point, "I don't need charity.") to the point of treating him like he is an outsider when her family encountered financial problem. This is a slap to the guy's face as he has considered her family but for her, it was like he wasn't. Also, she feels like he was only pitying her that was why he was with her. (That's what I really want to know too, if why does she think that way?"
For those who are still very confused on what Cheng Zeng has written on the book at the temple, all those years he visited the temple, he only wished for "ONE" every time. He always write "Su Yunjin" her name, meaning his only wish is to be with her, to be loved by her but wasn't reciprocated for all those years.
Maybe it is really true, "you only realize the importance of someone once they're gone." As she realized who is Cheng Zeng in her heart, she flew to the U.S. to look for him. As the title of the movie itself "Never Gone", he still loves her no matter what.
What I also can't understand up till now is why can't she fully give her heart to him for those past 6 years they were together? What was holding her back? May someone can explain too cause I'm confused. All in all, good movie to watch.