Screenshot everything she's sent you, then block her number/on accounts she's harassing you from and as others have said, contact the police.
From what you've said it sounds like the guy you're seeing has not been totally honest with you, and possibly his ex too about things. Either way this is all far too much drama and too full on for so early on. If I were you I'd just move on; like you say you're already resentful of him. What's the point continuing with things then; you have nothing to fight for?
I hope everything goes OK with the police and you can get everything sorted.