How to forget jealous and how to trust.

Hi ladies, I'm so stressed right now and I know you could help me.
Me and my bf we had a relationship for about 2years then we broke up; even though we were separate, we would talk almost everyday, and after a year we got back together.
When we got back, I asked if he had sex with other girls during that time and he denied... Even though I knew he dated some girls right in the beginning when we broke up.
But I found some messages in his phone that clearly says he had sex with another girl during a trip.. And that makes me really sad.
Not only that I get jealous but that he lied to me :( 
I know that he wouldn't cheat on me, but if he lied something that happened when we were apart, imagine something NOW. I kind don't know how to trust him anymore.
Please , could someone give a advice about it? I don't want to feel sad or stressed about it.
And I don't want to tell him that I read messages in his phone(he didn't know I did that)