In 5 Centimeters per Second, why was Takaki struggling in love? It is quite an old movie. Still...

SPOILER ALERT

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?
I do not think this has a definite answer.  5cm was about love, growing up, and all that entails.  So in truth only Shinkai knows exactly what was meant.

What I can offer is what I felt was Takaki's issue.

He never quite got over the fact that he failed to send that letter.

In my view, it could have been a self-imposed sense of not worth being in a relationship, or other such things that we do to ourselves when things do not work out.

in other instances he seems to be getting into a relationship trying to fill the emptiness left after losing his first love.  And that is a mistake that a lot of people end up making.  The end lesson in life, is that you cannot fill a void left by one person with a different person.  That void will always be there.  One just has to learn how to live with the pain that happens in love and life.  Takaki does not seem to do that very well.

My answer might not be the kind one can be backed up with research, and you may not agree with it, but it is the only answer that I believe can be given for this specific movie/book.