11 months and he has cheated and lied

I have been with this boy for nearly 11 months now and at the start he seemed like mr.right!! When i met him i was a virgin and i lost my virginity to him. Im not the type of girl who would loose it to anyone it had to be with the man who i wanted to be with forever. We got so close and he was more than just a boyfriend he was a bestfriend too.However some of him mates were heavily into drugs but he swore to me he had never touched them. A few months later he admitted to me he had done drugs and this hurt me so much because why did he lie to me? Then another day i was over his house but he hadnt come back from work yet. I was just browsing on his computer when i found a folder of msn convo's. One of these conversations was with a girl that he was trying to get with just before we got together. He said in the convo that he didnt think we would be together for much longer and that he wanted to meet up with this girl.This absolutley broke my heart. I couldnt believe it. I had never cheated on him or even thought about it. I had never even looked at another boy i had no reason to beacause this boy was my world. When he found out that i knew he tried denying it but how could he?? It was on his computer!! He broke out into tears and told me he was sorry and he didnt know why he had done it and that he loved me. When i looked at the date of the conversation i realised that i had stayed over his house that night and infact i was in the same room as him when he had spoken to this girl. This made me even more upset. How could he do something like that especially when his gf was there in the same room as him. Its taken time but i suppose i am gradually getting over it but i can never forgive him for what he has done. Also i understand men will be men but i asked him a genuine question about has he got any porn. He told me no and that he wasnt interested in that type of stuff. I believed him as when i first met him he didnt seem the type to just think about sex. However another day wen i was on his computer i found a folder full of it. I didnt care about the porn being on there i cared about him lying to me. I don't know if i can ever trust him again like i used to but it hurts so much because i gave him my world but it obviously wasnt good enough. What more did he want??