So sorry for the stress and ordeal of what you and your husband are going through. Hopefully this is a blessing in disguise.
This happened to DH & I almost two years ago, finding out that he had a daughter. My DH & I had been married 11 years at the time, second marriages for both of us. I have three children from a previous marriage, all adults now, he had no children with his first wife.
I was on FB and noticed I had a message from someone whose name I didn't recognize. She asked if I knew ......... From her profile picture she appeared a young adult. I was like, "and, why do you want to know?" Of course, all kinds of crazy things are going through my mind. She says, "I think he is my father"! Yikes! Then she goes on to say she is over 18, she doesn't want anything, she just wants to know where she came from. She tells me to ask DH if the name ........ means anything to him. Well, as soon as she said the name, I knew she was probably his daughter, as DH was stationed in Korea in the Army, and had dated this women during that time, and he had told me about her during our getting to know each other conversations.
I go out and ask him if he remembers ......, and he has a blank look on his face. I said, does the name Heather mean anything to you (daughter's name), and he says no, the only Heather he knew was ex-wife's cousin. I then proceed to tell him I think he has a daughter. He turned white as a ghost and was in complete disbelief.
She told me all about her likes and dislikes, and it was eerie how similar she is to DH. She is also built just like him. Anyway, we found out her mom had gotten pregnant right before DH was sent back to the US, and she never let him know. I have no idea why. Apparently when Heather was about to graduate from HS a few months prior, and enlist in the Air Force, her mom was looking at her graduation picture, and thinking about how much she looked like her father, my DH, and broke down and told her.
So, after the initial shock of finding out, DH called his daughter, they had a nice phone conversation, and the mom asked if she could call DH. She has since been married a few times and has four other children, and is still married to the father of the youngest three. She never told DH why she didn't tell him about Heather, and DH has never asked. He felt it wasn't important at the time. I will never understand how men's minds work!
Anyway, he met Heather a few months later, after she was home on leave, as she lives a few states away. According to DH they had a great lunch, and spent several hours talking. DH brought pictures of his family to show her. Heather's mom sent DH tons of photos of Heather growing up, via email.
You would think this has a great ending, but after a year or so had gone by, with DH sending Heather texts weekly, and calling her occasionally, Heather has basically stopped communicating with DH. It pisses me off, as well as breaks my heart.
She was medically discharged from the Air Force due to migraines and fibromyalgia, and has decided to stay in North Dakota since she met her significant other a year ago. I guess all she really wanted to know is where she came from. I'm hoping it's just immaturity, as she is only 21 now, but I know it hurts DH.
I hope everything turns out wonderful for you, OP!