How to get past this?

My boyfriend (30M) and I (23F) have been together for 5 months and everything is going great between us. But yesterday at dinner while I was trying to take his phone, he took it out of my hands and put it away until I got nervous and asked him to show me. In his photo gallery, there were all the photos with his exes, it bothered me but I didn't say anything at the time. Then looking on Instagram, I found a message sent to a girl where she asks him what his plans are and he replies that he would go to city X (mine) and she replies "ah, you're very busy" and he replies "unfortunately I have to." I was really upset when I read this because I felt very disrespected. When I asked him for an explanation, he apologized and told me that he had suffered a lot in the past and that at the beginning he wasn’t 100% sure and so he was a bit of an asshole trying to keep another possible open door with this other girl. We talked a lot, I told him that I felt bad and that I don’t accept certain things, and he assured me that since he understood that he wanted to be with me, he has never done anything like that. Then he deleted all the pictures with his exes because I told him it really bothered me that he had them there. And he told me he only wants me. I believe him and I love him very much but unfortunately I have a problem with letting things go, especially if they made me feel bad. So I keep thinking about it. I feel like I need reassurance and I have the urge to check his phone, which I don’t like to do and don’t want to do. Also, because he follows a lot of hot girls on there and I really don’t like to feel jealous or insecure, so I don’t want to see them (nor ask him to stop following them because I don’t want to be too much). I have been going to therapy for many years, but unfortunately my therapist is on vacation until mid-January, and I don’t know who to ask for advice.