It is quite an old movie. Still...
I finished watching the movie and manga a few years ago. I've just finished the novel by Shinkai-sensei.
After watching and reading through these, I completely can't figure out why Takaki's relationships couldn't last.
He had 3 relationships, 2 during his university life and 1 with Mizuno. Let's just put the first two aside, I think he's going well with Mizuno. But, at last, when he took Mizuno to the station where he and Akari met each other again, he just couldn't and cried. Why?
Because judging from the novel, it seems like he was not trapped in the memories with Akari when he was going out of Mizuno, not to say that he'd forgotten the memories.
In my opinion, the last scene, where Akari and Takaki passed by each other, only Takaki was relieved and really started to move on.
But what really was Tataki's struggle?