There's a very great deal to unpick in what you have written. I will only say... it is no wonder you feel confused a lot because much of what you say about your relationship with this man, and about the man himself, is totally factually incorrect. For example, he is in no sense whatsoever a good husband or father; he is an extremely abusive one - and this is just one example. You need to cut him off completely and only when this line has been truly drawn will you have the beginning of a full, healthy, fulfilling emotional life. You have terribly wasted many years and you have done enormous damage to the lives of others, even if that damage has not made its appearance yet. Assuming his wife doesn't know yet, when she finds out it could destroy her life, his life, or their children's lives. The consequences could be as small as a complete explosion of everything they have built materially, or as substantial as a suicide. The less present you are in their world at that time, the better. I do not think you need to explain a thing to him, frankly; it was always a given that your relationship could end at any moment and you can decide it's that moment whenever you like.Once you have some distance from it and hopefully some experience of a real relationship with a decent adult, you will have a different perspective on everything you have written above (and everything else you haven't written too). I fear it will not be pretty and the things you come to understand will be quite painful. I am really sorry you are in this rather sad place. Then again of the three adults (and however many children) involved you are in the least bad place for what's around the corner. The sooner you start with as clean a slate as possible the better.
One of the really hard things about being in a clandestine affair is that it is secret by definition, and so you can't gain the perspective that can come from dialogue with objective friends. Counselling may be the best way to address this problem. Good luck.
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