I just want to blabber a little about the theme of control in Someday because 3 of its four characters have huge control issues.
First, we'll take the most normal person out of the four - Haeyoung. Haeyoung is neither a control freak nor is she powerless. Her attitude is perfectly healthy - she does not go around thinking it's her fault somehow that Go prefers Hana (because ultimately she cannot control the way someone else's heart would go) but nor does she view herself as a passive victim of fate - she does try to change his mind - but not in an evil other girl way but in a sane normal fashion.
Unfortunately, that normalcy is something the other three characters lack.
Let's star with Go. Go's problem is that he is good at exercising control and so he does it as much as he can, even when he shouldn't. To put it plainly, he's controlling - Hana comes over to Korea to solve the mystery of Gumiko's diary. She does not come to find her mother. Yet Go insists on finding the mother for her, thrusting himself front and center into a complex emotional situation. Why? He is not Hana's boyfriend or even friend, but even if he was, that is no excuse to do things, major things affecting her life, without her consent or even knowledge. And it continues - whether it's huge things like finding her mother or bringing over her grandmother from Japan, or little things like taking her to a cemetary without asking her first, or proposing to her/pressuring her in a crowd of people, even while he knows Hana has no interest at all. There is a huge problem with that, aside from the fact that it is simply wrong - whatever benefits it may give him in the short term, it's an awful long-term strategy. Does he really think Hana (or any other person) would want to stay in a relationship where she is constantly controlled and/or manipulated? This is going to doom anything with Hana to disaster! He should know this uniquely well - the man is a shrink by profession and can read others like a book. But yet it seemingly never occurs to him things won't go according to his plan longterm - we get the sense that he's someone who's never had a wish of his denied or a desire ungratified. I think he may actually not care about the longterm because he's so consumed by immediate gratification, so unused to defeat. He needs to learn, so badly, that just because you can control someone, doesn't mean you should.
Hana has a different control problem. Her trauma is that her mother walked out on her to be with a man when Hana was still a child. Hana, thankfully, did not devolve into blaming herself or anything else of the sort, but how she dealt with the situation, with maintaining control and not being hurt again is to precisely delineate what is worthy of control and what is not - and she made her view on love be that it's a hormonal delusion at best, clearly it was something nonexistant and thus not worthy of her, Hana's control. Thus if Hana's mother left, it was an insignificant irrational issue, not something to make Hana feel powerless or hurt. She couldn't control it, but that was because she didn't want to. It was not worthy of Hana's notice. That is safe, yes, but it's horribly limiting. What Hana needed to learn (and by now she has) is that some things are worth trying even if you can't control them, even if they have the capacity to hurt you. Loving is worth the risks.
And that brings us to Seokman. For Seokman, he needs the feeling of control (even if that control is directly tied to guilt) in order to function. His whole life is defined by responsibility - he genuinely feels the accident that killed his family and put that bike kid in a coma is his fault. This is patently absurd - he was in the back seat and he asked his father to speed a bit because he wanted to catch a game on TV. Leaving aside the fact that his Dad was the driver and it was up to him to listen or ignore what a kid is saying, there is the big honking fact that Seokman cannot in any way be held responsible for the fact that a truck driver fell asleep at the wheel and drifted into the wrong lane. But Seokman has to feel it's his fault in order to have control over his life, paradoxically enough - because if it's not his fault, then he's just a victim of some really awful luck and it all becomes meaningless and he's just terrifyingly helpless. By claiming responsibility, it's his way of reclaiming control (sort of the way victims of crimes often do the blame game - "if only I did not walk down X street after dark, as I should have known not to, I would not get mugged." It restores power and agency to them). Because if it's his fault, then he's resonsible for bike kid's coma which means he needs to take care of bike kid's medical bills which gives him a reason to live. So very messed up. Basically, what he needs to learn (and he has started, already) that sometimes you need to acknowledge you have no control over bad things, and the best you can do is grieve and let go.