I need advice, I need to vent.

Today while helping my boyfriends daughter clean her room, I came across a bag that had letters and cards. These cards were from his ex girlfriend, including a letter that he wrote to her begging her not to leave him and whatnot, he was expressing his love to her. I decided not to keep reading, although this was in the past, it hurts to come across these kind of things. I decided not to say a thing but his daughter went ahead and showed him the bag and asked him if he wanted to throw it away. He looked at it and asked her if I had seen him, she said yes. He then went ahead and told her to throw them away. I haven't spoke to him about it and he hasn't mentioned it, but he can sense I'm upset. In the past I've also came across pictures he still kept of them both in his computer and he also went ahead and deleted all that. Before him and I met, he was living with her and apparently she left him for whatever reason it was. What bothers me the most is that I moved in with him after he moved to another place, and he still decided to bring those things with him. Ive been taking things easy and I tend not to overreact about it, which he has mentioned and gets surprised that I only mention it once and never again, and also move on. I don't like to fight, but I've reached my point. I'm also pregnant, so it hurts a lot more that i come across these kind of things and I don't know how I should be reacting to it. He's very good to me but also has a really bad habit of yelling and being mean when fighting. I on the other hand am the complete opposite. I just need some advice....