I'm really glad you liked it and found it so moving! And, god, I'm glad the final scene worked for you and others, because it really was supposed to be something else, and I felt what I did instead was just going to fall flat.
But yeah I like writing internal feelings a lot (whether that's in the emotional or mental or physical realm) because they occur in this physical place that exists but because it's filtered through the brain and the mind, you have the freedom to write about it in metaphor, in image, in strange or idiosyncratic ways.
Also I'm really really happy that you read the letters! It felt like so much to ask of anyone and I was not sure about their inclusion at all until after spending weeks writing them and being like, damn, I think these all gotta go in.