This is a massive betrayal of trust and I don't think such behaviour is necessarily related to sex addiction. It sounds more like he's been very unhappy for some time and instead of behaving like an adult and working through that; he's gone behind your back and betrayed you, the sanctity of your vowels, and your whole life together. One way or another you need to confront him. Print off the emails and any other evidence to lay down before him. Ask him why. Make it clear that if feels he has a problem he MUST be prepared to seek help and not only work very hard on those issues but also on a marriage that is now in crisis. Only in his response will you know what to do next but if he's vague, largely unapologetic and unwilling to change - don't waste any time. After years of lies, deception and downright mistreatment - you don't deserve another minute of it.

I'm so very sorry this has happened to you.