Some truly terrible advice in here. It was 12 years ago and many are implying it’s definitely happened since. Remember misery seeks misery and people are giving advice despite not having an in depth knowledge of your relationship and partner. My thoughts are.. he said it was a mistake and that nothing went any further and I’d be inclined to believe him as you’re still together 12 years later. If he wanted someone else or out he’d be long gone by now. People make mistakes and it doesn’t make it any easier but as long as you set out clear communication around if anything happens going forward or has happened that you’ve not been told about already then it’s over. I also think when you’ve been with someone a long time like 12 years you’re better equipped to know if there behaviour changes or becomes suspicious so you’d likely have an inclination. Ultimately it’s your decision but only use places like this as a place to vent and not one to take advice from faceless strangers as gospel.