<human>: Condense the text into a few key points

- 1 month ago my partner (Ursula) started acting suspicious with her cell phone, in that, she wouldn't get off of it. (She was texting her cousin's partner (Yzma, 37f) so I assumed this meant the relationship was safe at least).

-  3 weeks ago I saw her iMessages on our iPad to Yzma talking about having a crush, and would Ursula be mad at Yzma if she cheated on me. I assumed they were talking about a 3rd party still - and actually called Ursula out. She ended up hitting me for going through her messages, not trusting her, etc.
Our relationship was obviously strained at this point, and Ursula spend a lot of late nights at her cousins house with Yzma. Again, I assumed it was safe, until I read up on her messages again. (I think she assumed I wouldn't read anymore?)

- 2.5 weeks ago I caught up - she'd been having an affair with Yzma. I love you's. Selfies in underwear. Lots of talk about sex they had had.
I kept this all to myself as long as I could - and was ready to move out. When I told Ursula this, and asked her again if she was cheating, she adamantly denied it and said she isn't a homewrecker, that she wouldn't do this to her cousin.
I ended up telling her about the proof. She ended up telling me that it was my fault for not paying enough attention to her. That I was also a liar for not telling her I could see the messages on the iPad still.

We live in a small, very isolated town that is populated mostly by her family. Leaving the town is out of the question right now because I need to finish getting a certification - but I could use some insight and help getting over this. She isn't helping at all, as she keeps blaming me. It's been 3 days since I told her I had screenshots of the cheating, and she's been verbally badgering me and saying she's not gonna give up on me. All the while, I'm too anxious to eat anything.
<bot>: GF of 17 years cheated on me with her cousin's partner, we live in a small town and I'd like some insight on how to deal.