Boyfriend going through phone

So my boyfriend recently came over and everything was normal then when he left he texted me "can I ask you a question?" Then he later asked about the random numbers on my phone. I then explained who they were. Then he told me he went through my phone and knew I was texting some guy. But this guy I was talking to only texted me a few times cause he was an old friend. He got pissed off. He has really bad trust issues and they were currently getting better. Until now I feel like he doesn't trust me. I feel like he couldn't even ask me in person. He faked being happy and everything. I've never cheated on him and I've tried to make this trust better. Idk what to do I feel so hurt. We have a future and a life together but it feels like he can't trust me... It's completely irrational for him to be mad because there were no bad messages. I'm trying to be calm and level headed through this. And no I'm not breaking up with him. I just don't know how to make our trust better. I feel like it's my fault he can't trust me. Please help.