How to get over flirting texting?

Looking for some advice on others who have gone through this...My boyfriend and I have been together for 2.5 years. We were in a long distance relationship that was on the rocks when I caught the flirting texts he was doing with someone in a different state. We broke up for a few months thereafter, primarily realizing how unhappy he was. Since then, we closed the long distance gap, and are both working hard to fix the important things. He's doing all the right things and desperately wants my trust back...however I am having a hard time letting go of his dishonesty. We've made great progress but there are few days that bring it back up. How do you let it go and trust that this he's a changed person because of this?