I've been married for 23 yrs. I just found out he cheated 20 yrs ago and keep it secret.

The other night after a few to many drinks husband confessed to an affair with a coworker that toke place 20 yrs. ago. He says it didn't mean anything and he has never done it again. Now looking back, he has always had a close friendship with different women in different cities we have lived. He says its only friends, but how can I trust its only friends, since he is obviously so well with secrets. I feel like such a fool. I never had an idea, ever. He used to say he would never ever cheat. How can I trust him again?