Relationship Advice

I've been really good about not trying let my last relationship affect or even bleed into my new relationship that has been nothing but positive! Then today after I saw/felt what was a trigger ( my boyfriend turning his phone and looking around making sure no one was looking) I went crazy and looked at his phone, I saw a vague text that could have been taken two ways, he was cheating or that's the way he talks with his friends, he was furious and completely upset that I betrayed his trust! He's done absolutely nothing for me to assume anything other than the trigger which sent me back to a dark place, how do I not let these things affect me!? How do I just completely let myself be vulnerable and just be happy and leave the past in the past!?