You can show your thanks in other ways than words. You
can use photos, artwork, or make a scrapbook.You can give her a small gift if that's allowed, or bake her something. If it's just saying the words that's hard, you can send an email or give her a handmade or bought thank you card.

I've told my T thank you in words, in a collage I made for her, in cards I've made, and through baked goods. She's always reacted graciously.