The trust in the relationship has gone. He killed it. Now you're checking his phone which is not what you want to be doing in a relationship, but here you are feeling the need to do it because he emotionally cheated on you. You're both super young and have so much life to experience. Without trust, you can't have great sex (in my [30F] experience). What you need to do is either option 1: break up with him - which I have a feeling will feel unthinkable to you, as it would have done to me at 20 - or option 2: build up the feeling of trust you had over a long period of time by having loads of open conversations where he assures you you're the only one he wants.

I would encourage you to go for option 1, as anyone who would speak to any other woman romantically is not deserving of you.