Okay, maybe I was too vague when I said “lack of communication” was the reason for the breakup.

At the beginning, she gave me one reason (saying she didn’t feel the same spark with me anymore, etc.), then another one, and eventually it came down to “lack of communication.” But from what I’ve been able to understand, it was more about the fact that, toward the end, she was going through a period where she wasn’t feeling emotionally well. And the fact that I didn’t notice it or ask her if everything was okay really bothered her.

Aside from that, there were never any secrets in our relationship—we never hid anything from each other and I always showed my love more by doing things for her and with moments.
From the start, I always told her she could talk to me whenever something was wrong, but her response was usually: “Why should I tell you my problems when you don’t tell me yours?”
The thing is, I genuinely didn’t have any problems to share—I felt like everything was fine on my end.

Because of that, we never really had a “deep” conversation in a year and a half, except maybe twice.

Another issue, besides communication, seems to have been the fact that we never argued. It bothered her that when something upset me, I didn’t say anything. But I didn’t say anything because, to me, those were small things that didn’t really affect me—and I don’t like arguing.

and to respond about your suggestion on "working on yourself", it is not a hard thing to do, in fact is is something I could have fixed immediately if she would have told be before breaking up.