Girlfriend (23F) did something that really hurt me (26M) and i have proof, but i went through her phone to find said proof. How do i aproach the conversation?

So, some days ago i went through my gf messages because she was being suspicious and to be fair, im really insecure.

She was texting a guy and normally she tells me about every new chat with every new person because she likes to tell me about those conversations. The day before going through her messages i saw said chat and she didnt mention it, quickly closing the app. Later that day, i saw it popup again and when i asked her she avoided the question. This, and seeing a heart emoji in the last message she had sent was what led me to investigate further.

Turns out, the guy was an old FWB of hers, he had a new account and she had reached out to him to ask him about it. To not give too many details about what came next, it seems like the last time they were in contact, before we started dating, they tried to hook up a lot and they were never able to do so. They had a 4 ish hour conversation about how sad and dissapointed she was about not getting to hook up with him and they were flirting the whole time. He even asked if it was right to be talking about it, given that they both are in relationships and she told him to stop worrying about it. In the end she was still flirting and he openly said he was horny by the end of the conversation.

This happened a week or two ago and to my knowledge they are still texting. I screen recorded everything and im not sure on what to do now, as i violated her privacy to gather this information, wich will probably go against me in the argument to come.