Your H arguments about he was not happy since 1991 is a typical argument of someone who had an A and tries to justify his/her action.
Marriage is a contract. If he/she is not happy and wants to end the marriage. It is understandable if both have tried their best and fail. Both can part and find their own partner. But by having an A while M and end the marriage that way is unacceptable morally.
In my case I don't need to tell the kids and they knew from the sms they read from my W's cellphone (while they were playing games). It hurt very much to them. Because it was their mother who committed A. I think you should tell your kids the truth. They are old enough to understand. Let them make their own judgement.
I wish it would not be like that for you. But sometimes life is full of up and downs. I wish you all the best.
