Advice...

So my best friend and I made it official. We were friends for about 7 years. There was always a spark between us, but we were both involved in very long relationships. I had completely ended my previous relationship right before jumping into this one. Now we're married and we're happy. BUT there's something that's killing me inside. A while back, only a few weeks into our relationship I had sex with my ex of 6 years. I have completely cut off all ties with him, but what should I do. How should I tell my husband. We as all make mistake so don't judge mine. Thanks in advance.