Betrayed and Scared of what to do

So basically, I’ve been having sex this guy for a while now ( 4ish months ) and we’ve grown pretty close. Until I found out the other day that he has a girlfriend!

They’ve been together since 2016 and he never told me or had any notes whatsoever on any of his social media (I found out through stalking his insta and seeing comments from his gf and stalking her).

I have no idea what the hell to do! I didn’t think I could get stuck in this drama but I feel absolutely betrayed! Do I tell the girlfriend? Do I hide forever?