boyfriend messaging his ex?

Marissa
Well. This evening I was given my boyfriend's phone to use Apple Pay to buy groceries, but while trying to get the card up his messages opened and I thought I saw the name of his ex. He had to finish work, so left me with his phone and curiosity got the better of me so I unlocked it and looked again. Turns out he's been texting her since February and while nothing looks super alarming, they made plans to hang out all the time. 
Now back if Feb he said he wanted to message her to add her on steam, so they could play games online. I told him it made me uncomfortable given their history and if he were to do this not to tell me because I don't want to know. He freaked and promised me he'd never message or talk to her. 
So now that I know he's been lying to me for a few months I feel so hurt and I want to confront him, but I don't know how. A part of me also feels like I should forget and another part feels like I shouldn't have looked to begin with and need to apologize for it. 
Any adivce would be greatly appreciated! I'm feeling really lost.