Service Animal… I got questions! 😩

So, my dog Bella is a yorkie mix. Literally is best behaved and trained dog but she only reacts for me. I have psychiatric disability. She goes everywhere with me lately with covid. I know that “emotional support animal” isn’t covered under ADA but “psychiatric service dog” is. But, for my dog to be a psychiatric service dog does she need my psychiatrist to write a letter? Anyone know? Please any information will help. I just want to get her registered and have the right paperwork for her to be able to be with me in all places.

Thanks 💜