This letters scene is a very interesting thing to focus so much on. Such a direct, "he'll be sorry once I'm gone," sort of moment, genuinely sad with a heady current of vindication: "You should have loved her."
I have mixed feelings. It's a wonderfully powerful scene for this sort of story, on the face of it (if, okay, a bit soap-operatic—but that's not necessarily a bad thing).
But it's also playing hard into some of the worst ideas that romance fiction can perpetuate: If you just do this or that, he'll finally come to realize that he actually loves you! This was never going to suddenly happen in their relationship, if it was as described (indeed, the author would seem to understand that, with the description, in passing, of his on-again off-again half-caring attentions, to date). The notion that someone like him is going to become suddenly properly appreciative when one leaves is a somewhat dangerous fantasy, implying it might be okay to get back together now when a bad ex-lover makes performative displays of penance. One should not leave a toxic relationship because then they'll finally truly appreciate you (and that's definitely a narrative that exists in our culture), but because it's toxic and you need to leave.
(Of course the story has the protagonist taking the sane and healthy option of, indeed, leaving... Honestly this is like a running theme with me and this series, that the protagonist does more or less all the right things yet I have something to complain about anyway XD)
Of course, contradicting myself a bit a lot for a moment, here, it's interesting to think of the reasons why he might have really turned around at this point. It's actually quite plausible. From such a point of view that, realistically speaking, forced arranged marriage is the real villain here. His abusive neglect by all appearances came from a place of having someone shoved on him against his will, which he did not handle gracefully or with appropriate consideration that he was one of two people in that same boat (and of the two, the one in a place of safety and agency). I couldn't blame him too much (relatively speaking) because the whole notion was shoved on him when he would've been too young to frame it appropriately, and it's plausible to think that nothing has managed to prod him into re-assessing what's going on (because humans can be bad at that).
If we're framing it like that, the whole thing is just a tragedy of bad societal practices, bad communication, and human failings. However, the story isn't framing it like that, that I can see, so I'm sticking with my original take, above. XD