I 28M found some messages between my GF 30F and another guy. Can anybody please help me figure out a way to approach a conversation with her?

I'm so sorry for the long post and I don't want to bore everyone with the details, but I'm kind of spiraling right now and I desperately need advice and want y'all to have the full back story. I 28M have been cheated on twice. Both were my first serious relationships, after the second time I basically gave up and just started casually dating. I never really got close to any girl and never dated anyone more than a few months before finding a reason to end the relationship. That went on for about four years until I met my current GF 30F. Long story short, we're both travel nurses and met in a city, dated for about 8 months, then went our separate ways. I always felt like letting her get away was a huge mistake on my part, and I considered her to be the one that got away. About 2 years later she reached out just to check in, and we both happened to be living in the same city again. So we decided to reconnect. I was so unbelievably excited and things have been going really well. We've been official now for a little over 5 months.

The only issue is, now that I am in a relationship, I find myself feeling so paranoid that she will be unfaithful. I started looking through her phone and insta just to see if anything weird was happening. (Never found anything) At one point she even caught me looking through her phone, and we had a really long talk about my history and how important being able to trust my partner is to me. She was very understanding and she handled the whole situation very maturely and compassionately. Despite that conversation I still checked her phone occasionally, this went on for a couple months until I talked to a close friend about it. He told me he thinks it's just my past trauma, and that I have no reason to distrust this girl and I need to stop looking through her phone and just decide to trust her, so that's what I did. Once I did that things started feeling better, I felt like my trust in her was building much more, and our relationship was deepening in way that felt really fulfilling. Things have felt almost perfect for the past few months.

Now to the current problem. This week I had a moment of weakness, and I looked through her phone. I found some texts between her and a guy that she's never mentioned to me before. (Lets call him Paul). They only went back to a couple months before she and I reconnected. And the gist of those messages was that apparently some guy she was seeing had ended things with her and she was now texting Paul about how sad and upset she was. Paul then proceeded to tell her how amazing she is, how she's "the blueprint all women should aspire to," how "she's literally the most perfect girl he's met." He then asked her if she's ever considered trying women, to which she responded "No, I like dick too much for that." The most recent message was from a few days ago, and it was him telling her he was only 6 hours away and really wanted to come visit her, to which she said "Yeah! Just let me know when!"

My first thought was that this may be a gay friend, but I still had a weird feeling, so I got on her insta and looked this guy up. I then checked their insta messages and found messages between the two of them going back for years. Like I only scrolled to 2022 and there were still so many more messages from before then. Based off the messages I definitely got the vibe that this is a guy that is very interested in her, and the messages are definitely mutually flirty up until she and I started dating five months ago. At which point she gets less flirty but she still reaches out to him every few months. There are several instances where he is being very flirty with her, calling her beautiful, jokingly asking for feet pics, and telling her how much he misses her. He also tries really hard to come visit her ,several times, in whatever city she happens to be in (at one point he even offered to drive 30 hours to come visit her, and she told him that was "too far of a drive lol." Every time him visiting just doesn't seem to work out, despite him clearly expressing that he's very willing to come see her. At one point he jokes about quitting his job in the Army and coming to live with her, and she replies "Haha yeah right, you still living with your girlfriend?" and he says "No she still living back home." So I know Paul isn't gay. Some of the other messages include her asking him to find her a guy to set her up with, him asking her about her love life, and them sharing gross poop stories. All of these spanning over the past four years.

(I'm almost done with the story, I swear.) I sort of freaked out when I saw these. I immediately texted my previously mentioned friend asking for advice. He told me that was really uncool of her and I needed to be very direct with her about the messages and have a serious conversation with her about it. I was petrified of admitting to her that I looked through her phone. So instead I just asked her leading questions like "Do you have a guy best friend?" She said "no." So I then asked "so there's no guys that you keep in touch with regularly?" At which point she said "She has a high school friend named Paul that she keeps in touch with every once in a while." She said they just talk about his job in the army, politics, and general life updates. I couldn't think of a way to keep asking about this dude without making it obvious I was jealous so I just dropped the conversation.

Now to the point where I could really use some advice. She and I are planning to start travel nursing together in May, and I am now seriously wondering if this is a bad idea. I feel like I need some closure on this whole Paul situation before I feel comfortable making such a huge commitment. I can not express how much I do not want to tell her I looked through her phone. Any advice y'all can give me would be so much appreciated. Normally I would ask my friends but I'm worried they'll tell me to just dump her, so I want to ask reddit before I start telling everyone in my life about these weird texts.

Thank you so much for coming to my TED talk, please please please help me out!