I had an affair almost two decades ago. Recently found out it resulted in a child

I am married to a beautiful lady. I love my wife with all my heart. We have 3 boys and I have a daughter from a previous marriage. We also have 3 grandchildren.Now this event is tearing us apart. I want our family to be together and happy. It was a mistake from so long ago. My wife wants to leave and I don't want that to happen I love her so much. Please help me save my marriage. Even my 3 boys want us to work it out. They love us both and want our family together. Please give me some advice or hope. Sincerely heart broken.