I discovered that my girlfriend (19F) was unfaithful.

Today, she was taking a nap, so I took the opportunity to use her phone to watch some YouTube. While I was on it, her friend sent her a text, and I opened it to reply. After that, I noticed she was chatting with someone else and had turned off notifications for that conversation. Curious, I clicked on it and discovered she was sharing intimate photos and videos of us together. I haven't brought it up with her yet. Any advice?