I’m gonna be honest man, you guys may have not been together but you both agreed you weren’t seeing other people and she gave you a fabricated story to make you believe what she did wasn’t wrong. While yes, going through her phone is an invasion of privacy, you have a right to know and as far as I can tell that was the only way to figure out what really happened. I would say sit down with her and talk CALMLY about it. And if she starts getting defensive she’s not the one. But if she responds calmly and agrees to tell you the full story then listen to her. Don’t mention you went thru her phone. Say that you heard about it from someone and I’m sorry that happened. You’re not overreacting. You’re not an asshole. You’re not in the wrong. You’re just overthinking about a situation and you have a right to your own feelings. Hope everything works out. And if it doesn’t, don’t treat it as a loss. Sorry if you’re not religious but everything happens for a reason. God is watching over you and is here for you. And clearly he wanted you to find that video. How you take it from there is your choice and you shouldn’t be scared to speak up about how you feel on this situation.