I don’t know what to do!

I’m going to try and make this short and get to the point. So to start off me and my boyfriend have been together for 2 years ( yesterday was our anniversary) and I trust him more that anything! I have never had anyone as close to me as he is. Anyways yesterday we were looking on his phone for a camper and his best friend texted him and when he backed out of his friends message I saw that his second most recent text was to a girl named Megan. I can’t even tell you how much I hate this girl, just writing her name makes my blood boil! I know why I don’t like her and he knows why I don’t like her, she flirts with him every time she comes to our house and a few months ago she randomly texted him saying “hey, I’m bored just needed someone to talk to” and he knows I saw the message and he hasn’t said much about it since then but like seriously she know we are together. Oh and by the way we are in the process of starting up a car garage and she is one of our client, that’s why she still comes around. If we didn’t need her business I would try and cut her out completely! But anyways He lets me use his phone anytime I ask, so I’m not really sure he’s trying to hide anything but to put me at ease I really wanna see there conversations. Is that too much or should I just trust him? And I feel like I’m going to get comments like if you trust him you wouldn’t even question him but it’s not him that I’m questioning it’s her and I know this might be hard to believe but he seriously doesn’t know when he’s being flirted with. When we first got together my dad had to tell him that I liked him. I just love him so much and don’t want to mess anything up! Please I need some help!