I’m not sure what to do.

Good day, I’m unsure in what to do. My girlfriend and I have been arguing lately, we never really did before except for why somebody wouldn’t pass the chips or forgot to do something, simple stuff you know. But lately it been over every little thing, I know on that I’ve been just as much to blame. She’s got this really good job but the head boss decided to put a bunch of people into a night shift, including her. Anyway to the point. Her team lead/superviser had been hitting on her and trying to make sexual advances. She didn’t really tell me about anything. After a week or so I was really deciding to question things and I broke down and looked at her phone, I know it’s wrong of me to do so but I couldn’t control my self. Anyway what I found was messages between them and you can clearly tell that a lot of it was deleted. However what was still there was a message from her to her supervisor stating “we can still flirt, but I can’t sleep with you. I love you but I’m not in love with you, I meant everything I said. I’m sorry”. Now this has been really bothering me. Her and I have talked about it but it didn’t really go anywhere. Normally in any other relationship I would have ended it right there, but it’s different. I genuinely love her. I want to marry her. I just don’t know what to think and the pain is eating away at my soul