Trust issues

Me and my boyfriend aren't married but we've been living together almost 6 years and have a 1 year old together. He admitted the other day he had gone through my phone messages and I feel so betrayed. I wouldn't have bothered if he looked for people unknown to him or guys (I have neither on my phone) because he was insecure or something, but he went through every single text message I ever sent, including my mom and my best friend Stacy too. He told me he couldn't help himself once he had started reading. Me and Stacy are really close and talk about everything, including problems. The messages were obviously never meant for him to read. My boyfriend and I are open and talk about everything, but a lot of things are sensitive. When I talk with Stacy about things I'm much more forward. We're both okay with talking about our relationship with our friends, but he became upset for some things I had written to her. For example I had asked for her advice because he is gaining so much weight and doesn't seem to care and I didn't know what to do, and my boyfriend got upset and claimed he does a ton of things. But he never told me anything about it so I didn't know. I obviously never told him "you're getting fat" because he's sensitive about it. I've gone the route of suggesting we eat more healthy because we need to think about our health and asked him to exercise with me because we both need it etc. Anyway, that was just one example. I get that it was hard for him to read my thoughts in all their honesty, but he shouldn't even have done that in the first place! So I feel kind of like he had no right to be upset. So now he's even more insecure because he read all those things and he's afraid that I don't love him and will leave him. And I'm making things worse with being secretive with my phone because I feel betrayed and I haven't been in the mood for being sweet and loving because of it. I feel like I can't talk to anyone about anything anymore because I'm so afraid he will snoop again and get upset. My friends all live far away and we all have different schedules, that's why we mainly text. I've been tempted to put a password on my phone to ease my anxiousness, but I think he would get very suspicious if I did. I have access to everything that is his, but I would NEVER go through his private messages with his best friends. It's such a invasion of privacy, like reading ones diary. Ugh, I don't know what to do.