It’s not even his fault. It’s me. It’s something in me
Yes, it is something in you. It's your soul, your personality, your heart. You don't have to assign blame here, if it makes you uncomfortable. People can grow apart, and it's nobody's fault, and it's hard, especially when one person has expectations and the other doesn't want to fulfill them.
But that's not a fault in you. That's not a fault. It's not your fault.
You say it's something in you as if that's a bad thing. Do you think you're not allowed to have feelings and make choices and change? Do you think you're not allowed to strive for things that would make you happy, and pull away from things that make you feel bad? You are allowed, and this marriage is not making you feel good.
So, yes, it is something in you, and it's something that is screaming at you because you always silence it, and you need to listen to it. Your children each have one of these, too; one of these hearts, souls, whatever you want to call it. A true self. Teaching them to listen to their true self, especially when it screams at them, is one of the most valuable lessons a parent can give a child. It will protect them from bad relationships, bad jobs, bad living situations when they are older. Stop reading divorce horror stories. Set your children an example: if you are unhappy and you feel the need to walk away, walking away is what you should do.