I don’t condone cheating, but I can understand where the guy might be coming from. Maybe he just needed to communicate more clearly with Rina about how he was feeling. If he wasn’t comfortable in the relationship anymore, it would’ve been better to break up first. Maybe he was just confused cause he found someone who fulfilled all his needs and wants without question. But still, that’s no excuse for going behind his partner’s back to cheat just to relieve his frustration. If he truly loves Rina and just needed space, he’ll end up regretting this scummy behavior.
Nah, to me it seems more so that after the initial idealised stage of the relationship, he's starting to notice the negatives or the things in the relationship they need to work on. This is normal in any relationship, it doesn't mean there's anything seriously wrong going on. But at the same time many people, especially teenagers, aren't really equipped to deal with that very well.
On the other hand, the new girl isn't necessarily "perfect"; she's just still new, so he sees her through the idealised lens. If he were to continue going out with her, he'd probably find things that would cause difficulties in that relationship, too.
He seems conflicted about his existing relationship and still wanting to be with her, so it doesn't look like a situation where he's not comfortable with the existing relationship at all. He just doesn't know how to handle that stuff and then meets a new girl that likes him and
seems more accommodating and he makes the worst choice out of it all.