Feeling all emotions a woman can feel

So last night I went through my boyfriends phone. I've never felt the need to go through his phone, because I trust him. But he's been acting fishy. We Been together a year and one month now. I found messages of him texting another female. They started texting January 23. I'm confused about it all because he always told me he wouldn't do that. But we talked about it last night and he told me a guy at his job told him to give me a taste of my own medicine that I gave him in the pass. He took up the offer and did this. Advice please.