Trust issues

After loosing some trust in a relationship how do you get over it and move on?  Recently caught SO chatting online in some group chat.. About sexual things , to this day not sure of exactly what it was but he claims he's not doing it anymore.  I want to move on and forget about it but it's so hard, always in the back of my mind & can't figure it out.  It puts a toll on our relationship because I get uneasy about dumb stuff now because I'm becoming paranoid. I'm worried about the future of the relationship. it is becoming a viscous cycle, me not trusting and him getting mad at me for being so parinoid! I need advice