Lying.

My boyfriend and I have been a very good couple and have gone through things most couples will never see in a lifetime. We have known each other for 3 years and have been together for 2 in September. I am 18 and he is 19. I was on Vacation for a week last week and have seen him twice since I’ve been back and am now leaving again. The time we were together was beautiful until last night. He decided to show me something on his phone and so I grabbed it with him being beside me and started to go through it just because we’ve had a lot of trust issues. I find a girl I didn’t even know existed and has been messaging back and fourth then deleting the conversations. I asked him why was he doing this to me again and he said he was sorry and it was just for school since he’s a firefighter and she’s trying to get her nursing license. I addressed to him the previous time he put other girls before me and our relationship, that if I find out he’s doing it again, it’s a done deal. I told him he no longer has me, grabbed my shoes, and left. I live about 15 minutes away, and by the time I got home I have message after message after message. I can’t stand a liar. I have always gone back on my word every time this conflict arises so we go in circles and we’re back here a couple weeks later. I guess what I’m trying to say here is, if it was just a question she had then why delete it? And how does she even get your information to began with? I know he loves me, I just don’t understand how you can do this to someone who has done everything in the world for you. I know I have my faults don’t get me wrong, but lying isn’t one of them to him. I’ve tried seeing it from his point of view and I always end up justifying why he’s doing it and making excuses when in all honesty, I don’t think he’ll ever change. If anyone is willing to give advise I’d greatly appreciate it. (The bottom pic is where it starts)