I have two suggestions.
The first is for the protagonist to find some reason to love the kitsune other than her appearance or some task she might do that the protagonist appreciates or admires. People often want to be loved for who they are, not because they make you breakfast every morning, have the fastest time in the hundred yard dash, or because their hairstyle is flawless.
This is a tough ask. You'll have to think about it.
Try not to use a laundry list such as, "I love you because A, B, and C," where A, B, and C are "you walk the dog when I get home late from work," "you know just how I like my tacos when we go to that Mexican place on 7th street," and "your thesis on snail darters was the best I've ever read."
Instead say things like, "I love you because of who you are. When we lay together on the beach last night, the stars and the moon shone brighter in the sky then they ever had before, and I felt peace in my heart knowing you were there."
The second suggestion is remember that letters are physical things, not just words. Material touches matter in a letter, such as the color and quality of the pen used (and maybe a second special pen just for a flourish on the signature), the quality of the penmanship, stickers and little drawings in the margins of the letters, dried flowers dropped in the envelope, and so on. Don't miss an opportunity to make the letter more meaningful by including physical touches important to the sender and recipient.