I relate to Princess Luna in MLP FIM the most, while I cannot say my hardships were as severe as hers, in the show her life was much harder than my own, despite the fact I don't come from a wealthy family, I have mental health problems, sure, but at least there are people willing to address mine and I received healthcare and medication for that. Princess Luna in MLP FIM on the other hoof, well, her problems mostly got ignored until it got really bad for her in Do Princesses Dream of Magic Sheep? it got to the point where she did not just think of self harm, she actually did it, and I genuinely felt bad for her last time I watched a clip from that episode, I watched the whole episode a long time ago, don't remember all the details, just bits of it where Rainbow Dash had that funny dream she hated, after that, Princess Luna and her friends dealt with the dream demon Tantabus. Seeing that clip of her where Princess Luna burst into tears from her self condemnation, though it was not a wise decision to create Tantabus, I still believe she deserved mercy, and she got it from her friends, showing how strong the sense of community is in Equestria, that is what I love about the characters and the show, I do wish Princess Celestia paid more attention to her sisters problems though, because that is what I believe indirectly resulted in their argument in episode Royal Problem in season 7, but even she is not perfect, I've seen fictional monarchs do far worse than her and I don't believe Celestia's behaviour was intentional, it was more the case of ignorance than spite toward Luna, however on the whole, from what I have seen of the show in other episodes as well, they help each other through their problems not out of expectation or ulterior motives, they are charitable, more charitable than humans are IRL, and they're not quick to start a war whenever there is someone causing a problem there with a shoot first ask questions later ideology, this was shown when Twilight Sparkle attempted to reason with Starlight Glimmer who was carrying heavy emotional burdens of her own in Cutie Remark, and that is why I wished I could reincarnate into Equestria to be part of that community, but it cannot ever happen. Yes, I would trust Twilight Sparkle to be a diplomat for Equestria, she seems to be good at neutralizing threats with strategy or psychological warfare, I could see a friendship happening with her too, and other members of Mane Six like Fluttershy, Rarity, Applejack, Pinkie Pie and yes even Rainbow Dash.
As for me and Princess Luna, we both feel bad for wrongs we have done, we both wished we could have done otherwise, but we cannot turn back time, only move on and learn. Goes back to the wisdom from Rafiki in Disney's movie Lion King "Oh yes, the past can hurt, but the way I see it you can either run from it, or you can learn from it." there is no greater wisdom here, it is what you do with the knowledge in the here and now, is what matters most, because the choices you make now, can affect the future.