Why is it so hard to trust a lover?

I have the sweetest sweet heart ever for a boyfriend now, but in the past I have been cheeted on several times and I can not get the thought out of my head that he might cheat on me which makes me weary and I want to look threw things to make sure hes not but I know that would be wrong and controling, how do I make my self feel better? I know he would never cheat on me so why do I ponder if he does?