How do you regain the trust back in a relationship?

I didn’t get cheated on, but he did do something that betrayed my trust in him bc he hung out with his ex behind my back. I know they are still friends and they were in the same friend group, so I told him that I‘m cool with them meeting up as long as he lets me know - but he didn’t. I only found out bc my friend saw them out together and sent me a pic as proof.

But ever since then I’ve been having a hard time trusting him and believing him. I think I just need to accept what happened and move on, especially since he hasn’t done anything else that would make me question his honesty ever since that last incident.

I need a mind hack that would help me forget what happened or at least just accept it… any tips?
How do you regain the trust back in a relationship?
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