Trust issues please help!

My fiancé and I have been together for 6yrs. We just got engaged a couple of weeks ago. Although I love him to death I have a hard time trusting him. Back in 2011 we had broken up for a few months but I just found out last year that he had slept with another girl. We weren't together then so I can't be but so upset. I just didn't like the fact that he lied to my face for so long. I know trust is very important in a relationship especially since we're getting married soon, but how do I start trusting him again? I'm always checking his phone, Facebook, and email and it's making me nuts! How can I start trusting him again??