why doesnt he trust me anymore?

this morning me and my boyfriend(or I guess I can call him my fiance seeing we are pretty much engaged), were looking at cars on craigslist and we came upon a dodge and I was like oh dave has the same kind of truck and he was like I dont “F” care why would you even bring him up and I was talking about a family friend and he thought I was talking about a guy that I dated for not even a month. I was like I was talking about a family friend and he didnt say anything after that and then somehow the conversation lead to him thinking I still talk to a guy that he doesnt like at all and I havent talked to this guy for like a month and I dont talk to him anymore because I know it bothers my boyfriend and he was like you lie to me all the time and I was like no I dont I hardly talk to any guys. why do I need to talk to other guys when im with the one that I love. I mean my boyfriend and I have been together for 2 years and I am so in love with him but we had a three month break up and he realized that I was the one he wanted to be with so he came to me and got down on his knee and gave me a ring and he has done nothing but prove to me how much he loves me and now im the one who is keeping my wall up and I think it might be because I know that he doesnt trust me. im not sure if this is true but when we first got back togethere he said if you trust someone completely, you lose them and he said he didnt wanna lose me. I mean c’mon I absolutely hate lying and thats the one thing that bothers me the most is when someone thinks that I lie. I have been in love with him for a long time now and it took me the whole 3 months that we were broken up to put a guard up over my heart so I didnt get hurt again and we have been back together for a lil over 2 months now and I havent let my wall down yet. not sure where exactly im going with this but I think you get the point?