You need to find out what that is. I'd suspect you have some idea. Use this as a wakeup call to work on the marriage. Maybe Relationship Rescue as well. Maybe one of those 101 Nights of Passion style of books too.
Point is, work on it.
Many of us are hurting on here, as you know, because we have been betrayed, but in a situation such as yours, where you are still together and want to make it work, you are in a good position relative to many of us, so make the most of it. Post here whenever you need to. You are not a bad person, just made mistakes. You can't change the past, but you can shape the future. Use what happened in the past to make the future wonderful.