Trust issues

My boyfriend recently changed his phone password and when I asked to see his phone to record something, he literally stood by me to make sure that's all I was doing. He says it's because he has my Christmas present on his phone, but I don't see why he can't just delete it. I saw he was snapping a new girl the other day. Not sure if I'm overreacting or should just let it be. I've gone through his phone before we were dating and we were just in the talking stage and saw he was messaging girls on tinder literally while I was with him. This makes me not want to trust him, even though I know were dating now. We've been arguing a lot lately and on the verge to breaking up, so I'm trying not to start an argument but everytime I see him messaging a girl I've never seen before, it makes me think he's up to something. He also had every girl I ever asked him about blocked from viewing his snap stories. He got mad at me when I brought it up and gave a 'reason" to why they were all blocked. I didnt believe his reasons and think he just didn't want them knowing he has a girlfriend. I don't know what to do.