I'm going to move into my boyfriend's grandmother's house
Why are you going to move into her house?
I haven't known her for long, but now I love his grandma more than I do him
I'm happy you're having such a great relationship with his grandma and you want to live with them, but it's not healthy to love her more than him. It might make your boyfriend uncomfortable.
She is the kindest person I've ever known and she's made my life better. I honestly love both of them.
It's great that you love both of them, and I'm happy for you. I'm sure you'll have a nice time living with his grandma.