Diane is supposed to be a girl that want to change the world. After hanging out with Bojack, she realised that she is way more like him than she wants. She knows she is not the person she wants to be.
I think it's more that Diane really wants to be a good person and change the world for the better, but she lacks the confidence and support structure to act on those impulses. Just look at how quickly she falls apart when goes to report on Sebastian: she lasts all of a day before breaking down(Sebastian himself is interesting: he's a self-absorbed twit, but the show seems to be suggesting that that's what enables him to do aid work in an active war zone at all). She wants to be this great investigative journalist, but encounters the reality that journalistic outlets don't want to do that, because it's expensive and disruptive and shallow click-bait is more profitable. And, of course, her relationship with Mr. Peanutbutter is (mutually) toxic.
I do identify with Diane a bit, because her internal monologue is familiar to me(Am I a good person? Shouldn't I be happier? Can I be happy? Does being happy in this garbage world make me a bad person?)
It's also that she has seriously bad issues with repression. She does not deal with her emotions, she does not talk about them, she does not face them. Going to Cordova should've been an amazing opportunity, but instead of resolving things at home first she used it as a way to hide from her marriage. Admittedly stems from a family dynamic that refuses to acknowledge any problems, or to take her seriously when she does.
One of my favorite episodes is when she goes to visit PB's brother, and every time they talk while PB is gone the brother is much darker and more somber. Same kind of dynamic done in a different way, but she totally fails to see how it relates to her.