Cheating- How do you trust your next relationship once you've been cheated on?

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I have been cheated on before and it hurt me badly. When we were together my family kept tellimg me something was wrong but I was so caught up I didn't listen. One time when I was gone he stayed at my house and my little sister went through his phone and read text messages between him and another girl, then she picked up the phone un the house in the middle of the night and listened to him talking to another female. She tried to call me and tell me but I didn't believe her. Then when we were at six flags he would always wonder off with this boy who lived with us and come to find out he was with him and 2 other girls hanging out. The thing that really did me in is this girl called me and said that he had told her that he was staying at his cousins house-(Which was mine)- and he asked her to come over because he was lonely he took her in my room and F*cked her. When I confronted him about it he told me that that girl was insane I ws like if shes insane how does she know what my room looks like? -Well that was the end of that. Now I'm engaged to the father of my child I have a problem with always thinking he's cheating on me. I know he's faithful and never has cheated on me, but its like I always think in the back of my mind maybe he's messin around on me. Another thing that helps me think those thoughts is his brother is a straight up cheater, so I know that if my fiance was to mess around on me his brother wouldn't tell me. I want to be able to listen to what my fiance tells me that he loves me way too much to do that to me, to take a chance of loosing me and his son. How do I put the painful past behiind me and trust what he tells me? I don't want to mess up my relationship now because of what someone else did to me in the past.