How to deal with a lying boyfriend?

My boyfriend and I have been together for a little over 5 months now. Everything up until this morning has been amazing. We're both very much in love with each other and if we've ever had problems it was only miss communications that were always talked out and solved. This morning I woke up next to him and since I had not woken him by getting up, decided to do something cute for him. I have cute backgrounds saved on my phone and I decided to send him one and then go into his phone and change his background, so when he checked it later he'd see a cute picture of a monkey holding a heart that said "you are my everything". Okay, to our none romantics, this sounds stupid but this is just something me and him do now and then, cute little **** like that. BUT in the midst of going into his texts to retrieve the pic, I came across an text thread from someone else.. I wouldn't have ever clicked on it if it didn't end with "see you soon, handsome :)" The number had no name. The conversation went with multiple references to her being beautiful and him also telling her that "he's trying the girlfriend thing right now, but it's a pain in the ***." She replied, "Why? I thought it would be a good thing." and he said "I don't have time for anything else. It's annoying." Just retyping this makes him look like a complete ***. But I'm telling you, I'm in utter shock. This dude literally told me hours before that he was 100% positive that I was the one for him and he'd do whatever it took to be with me forever. He also told her that they should hang out soon. Too be honest, I have no idea how to deal with this. I know I need to confront him but what do I even say? And regardless of what he says, does he deserve for this to be a forgiven incident? I have no idea. Please help. Thank you.