I found texts on my girlfriend's phone after she lied about where she was. Did I do something wrong or was this always going to happen?

Me and my girlfriend have been together for about eight months. Things have been really good. No big fights, no drama, we actually communicate well which I know is rare. Last Saturday she told me she was having a girls night with friends from her old school. I had no reason to question it. I said have fun, she said she'd text me later, that was it. We share location with each other. We set it up early on, both of us agreed to it, it was never a big deal. I wasn't even checking up on her. I opened Snapchat to look at something else and her location just popped up on the map like it always does. She wasn't where she said she was. She was across town at an address I didn't recognize. I told myself there was probably an explanation. Maybe plans changed. Maybe I was misreading it. I texted her casually asking what she was up to. She said still at her friend's place, just watching movies, pretty low key. She was still across town when she sent that. I called her and asked straight up where she actually was. She got defensive really fast. Said why are you checking my location, do you not trust me, I can't believe you're doing this right now. Then the explanation came. Said the plans moved to a different friend's house last minute and she didn't mention it because she knew I'd make it a whole thing. I couldn't sleep that night. I went back through my phone looking for anything I'd missed. And I found it. A whole thread of messages between her and a guy I didn't recognize. Nothing explicit but enough. Inside jokes. Late night texts. The kind of conversation you don't have with someone who is just a friend. I confronted her the next morning and showed her what I found. That's when everything came out. She had been talking to him for two months. She said nothing physical happened but that she had feelings for him and didn't know what to do about them. It was the worst fight we'd ever had. She was crying, I was furious, we said things that can't be unsaid. By the end of it there was nothing left to save. I ended it that night. It wasn't just the lie about where she was anymore. It was two months of something I knew nothing about. It still doesn't feel real. But I know I made the right call. I just wish I'd trusted my gut sooner because looking back the signs were there.