How do you react to this. How do trust them again? Can you trust them? How do you make sure you are not fooled again without being over controlling?
She has to EARN your trust. That is the only way it will work. You asked what your mistake was, well it's pretty simple, you trusted her too much too soon. FULL transparency means exactly that. You should have her IG and FB passwords and whatever other social media account passwords she may have. You should have full access to pop in to those accounts whenever you want to verify she's not having contact with any OM. This is NOT "over controlling", it might be in a healthy relationship but yours is not a healthy one. You trusted her and she blew it not once but TWICE. The only way back from that is for her to give up some of her privacy to prove to you that you can trust her again.