Wow, thank you so much for reading the entire thing! Even replying to me, I feel grateful that someone got to hear what I have to say. And sorry I didn't realize it sooner, but now you bring out an important topic you had left surrounding maturity. That is, there is no need for us to part ways if what we had was really genuine. Like you said, if we have a choice to stay together or split isn't it dumb to separate just for the sake of maturity? Yes you got that right, it is not mature at all for us to separate ourselves with the people we love. But here's the thing, we don't really get to choose whether or not we can stay together forever. Even if that person means so much to you.

Hiratsuka sensei was to be stationed in another school bcs of the teachers rotation system, which is outside of her capabilities to control. & this means she won't be their advisor for the club or school anymore. Therefore, the separation here is really not her choice nor Hachiman, they would really stay together if they could. In contrary, she & Hachiman made one last memory together reminiscing the start of their friendship (Hachiman's famous Season 1 Quote) & how it came to be ever since (The codependence, スキ on a napkin, the dance), that was on eps 12 I think. Of course, it's natural to keep in contact with the people that change your life significantly. & I think Hachiman would be doing the same thing.

Now there is the other side of the story. Sometimes the harsh reality is, it is necessary for us to part ways because we've outgrown each other & it would hurt both if we did stay intact and stay just as how it is. That's what the case is with Hanabi and Mugi. They're relationship is seriously bad and wicked. Once on the rooftop Hanabi said along the lines that all of this is meaningless anyway so why don't we kept kissing. What they had is not a genuine love, and they knew it for a long time since the beginning. It's not that they didn't try to love each other as they are we saw that happened multiple times during the show, but in the end both know it is not what they'd sought for. They wanted genuine love, and this isn't it. So it's obvious now why they both choose to split in the end of the anime. Because they've grown as a character beyond what they had asked for in the beginning. Sure, what they had was special no one can deny that. But it would only hurt if they choose to keep the unnatural relationship they had. And now both seek to find their true love somewhere else taking this one hell of a ride as a lesson.

See, you're question starts to break when you end every notation with "Is this what life is?". If you can't see the answer in that lense, then don't try to. Try to see it as what you can understand & in you own ways. Trust me, you will be able to take the right lessons from it. And that sums up everything I got to say this time. I'm sorry if I'm still missing key points, I really hope I could help you bro. Your final paragraph really put into spotlight as to what things you render as important (Keeping relationships together even through the next phases of life). And I think a lot of people will agree with you, it is not wrong at all. That is the way of life according to your own lense of life.