Is he going to ever trust me?

How can I reassure my boyfriend that my lapse in judgement in talking to an ex and still having an emotional conversation with him, was just that...a lapse in judgment. I was confused and was having a hard time with the fail of a relationship. I never meant for it to hurt him or to do him wrong in any way, and i know he's the one for me, and i am afraid he won't trust me anymore.