I don't understand how someone just decides they don't love you and "aren't happy" after 12 years of being together.
Your husband is one of many, many narcissists out there that crave attention and validation.
It’s literally drug-like in the dopamine/endorphin rush that they get from people in the form of praise and attention.
People like this will act in whatever way they need to in front of whoever they are with to appear ideal to them in order to get the praise and complements they crave.
You were the current drug of choice for him for a number of years but, after some time, the drug lost its potency and the effect is not enough to supply his need for validation.
Along comes a new drug in the form of fresh, new female attention.
Like any drug addict, they will drop the old drug for the fresh new high of the new one.
He doesn’t get satisfaction, worth, and validation by living authentically via values such as honesty, loyalty, commitment, and compassion.
He gets it from fleeting praise from people that validate his fragile ego’s need.
What you must know is that his disloyalty and betrayal had nothing to do with you or your relationship/marriage.
He would have done this to anyone and is likely to do the same to his current addiction.
Be glad you did not have children with this kind of person.
For the pain you may feel now is 1000X worse in the pain you feel for children having to be put through the cruelty and trauma of having to watch their family being destroyed from within for no other reason than someone’s inconceivable selfishness.
I’m guessing you are around your husband’s age - about 30.
If so, then, as much as it hurts right now, you are fortunate that this happened at a young age.
There are many here that the same thing happened to them after decades of marriage.
His betrayal now spared you the pain of it happening late in life - although, even later in life, it’s never too late to live a better joyful life after leaving the misery of this kind of betrayal behind.
I would bet a good amount of money that at some point in the not too distant future that you will look back and be glad this pathetic woman helped rid you of an adultery time-bomb waiting to go off after who knows how many years of marriage to him.
My best of luck to you.
You will be fine and happy.