Birthday card proofreading?
Hi, I'm a couple months in to learning Persian, and I'm trying to write my girlfriend a birthday card in Persian. I'm confused by some of the writing vs. speaking conventions, and I was wondering if someone could proofread it? It's not very long since I don't know that much Persian yet haha.
لیلا عزیزم، تولدت مبارک! بیست و هشت سال داری. تو بهترین دوستدختر استی. خیلی زیبا استی. دوستت دارم، لیلا جان. با عشق، ایوری
(What I think this says: My dear Leyla, Happy birthday! You are twenty eight. You are the best girlfriend. You are so beautiful. I love you, Leyla dear. With love, Avery.)
I know in speech, you would say "you are beautiful" as زیبایی, and I've heard that هستی is the spoken form "you are" and استی is the written form. But I'm confused when do use which version (written vs. spoken, formal vs. informal); when do you use زیبایی vs. زیبا هستی vs. زیبا استی?