Why am i snoring and how do i stop? i am in perfect shape. i've had tonsillitis before. i sleep with my mouth open. i'm very healthy

Snoring/OSA: Assuming you are a thin or fit young man as you say, then you likely either have large tonsils (which you would know - they are tough to hide) or else chronic nasal obstruction likely from a crooked septum or (much less likely) retained adenoids behind nasal cavity. See an ENT to do a good endoscopic exam of your nose and throat and decide if you need a sleep study.

Snoring: The reason for the snoring needs to be determined before a fix can be identified. Sometimes a sleep study is needed for this reason.