Should I trust him, trust that he actually is willing to change?

Been with boyfriend 2 years. He gave me his password for Myspace and I never even wanted to check it except for when he went out of town for work. I read what he sent to 4 girls that we both know. All the same pretty much saying "Hey you're a crazy girl, when are we gonna get f**ked up ...you're such a cool girl call me sometime"...I confronted him he said he was drunk and bored. I thought nothing of it. THen I have noticed that he had been looking at porn and other women on myspace whenever he was alone. He also would lie about when his girl friends would call because I dont like them so he didnt want to make me mad. This went on for a long time-if I wasnt home or if he was out of town for work Id check his history and see the same thing-he would check out woman on Myspace in the area where he was or girls we both know that are huge sluts. I had enough so I broke things off and stayed with a friend. The night I left I noticed that he looked up women in our area on Myspace and friend requested one. Then he was out of town for work and was looking up women in that area where he was. Seemed to me like he was looking to hookup. He says he had to get his mind off of me so he was looking at women, but not intending to hook up with anyone. I dont believe him. We are back together now..things are going wonderful...but I keep thinking that he's still going to be looking at other women and cant fully trust him. Im so in love with him and yet so confused. Wonder if I did the right thing by getting back with him and wanting to try again. He says he wants to change and really is trying hard and knows its not right to look at women like he was. know he is trying hard, but I know that he is aware that ive checked the history so he knows to delete everything if he would do something that wasnt right...I just dont know...I want to fully trust him but theres that voice in the back of my mind saying Im an idiot for giving him so many chances. He says that when he looked at women it was because he didnt think I was that interested in him or it was his way of just getting stuff off of his mind, but then why not just look at porn and why search for women that are right in our area where we live or where he was for work?Thats really the story....long version cut short.