He didn't deserve to read a letter about her love and devotion to him.
And his flashbacks where he's acting like he was powerless regarding how she was treated? Fake news. She was under his and his family's care/power for 10 years. It's like being a parent and being surprised to learn your toddler didn't eat all day. 100% it was his and family's choice how she lived in that house. It doesn't mean he has to physically prepare a room for her but he is responsible to overseeing the servants and aids are treating her right. In the letter, she's praising his management of the territory. So we know he has common sense and understands his duties. There's no excuse.
Instead of crying about her love letter, he should feel her accusation of neglect. Because it's deep. In every way and up to that last moments of their engagement, he neglected her well-being physically and emotionally.
Anyway, I love stories like this where we don't have to see 20+ chapters of how she was neglected and mistreated and disrespected. Misery p**n. All the shade to remarried empress.