So I get what they were going for her, and as others have said, communication is key, but honestly I didn't much like this confession.
This series has not been that emotionally deep or dramatic thus far and while we've always known he had self-esteem problems caused by what that girl said to him when he was young, in my opinion we've had too many chapters (nearly a hundred) of him being well-enough adjusted. Not super outgoing, but not freaking Shinji. Taken in that context, acting like this suddenly was being a MASSIVE dick, and for two high schoolers, that to me is a big turn off.
Once you're married, you put up with some pretty dramatic stuff because it's what you signed up for, but when you're high schoolers (and I think they'd both be each other's first partner?) and someone goes this dark this fast and takes it out on you, that is a big red flag. RUN.
...and that's all before it's revealed that the reason they acted that way was JEALOUSY. Jealousy as a motivating emotion to treat a partner like crap is an incredibly bad sign.
I'm gonna finish it because the story is almost over and because I assume they'll recover from here on, but if I were Marin, rather than being over the moon, I would have had a very serious talk about acceptable ways to deal with emotions when starting out in a relationship. I've seen way too many peppy girls who think they're going to fix some moody guy find themselves in a stressful, emotionally manipulative and potentially even abusive relationships to be happy at a confession following this sort of behavior.
It would have worked better if he'd volunteered to explain why he'd acted that way and apologized without her having to force him into it.