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Maybe you married too young, which many do. And maybe you missed the opportunity to find yourself through a period of expression, experimentation and freedom. Maybe that was a significant blow to your identity. But the fact you wrote this letter suggests that you're already halfway gone, so go already.
I'm not sure he'll care about your freedom while his marriage shrivels up and blows away. So do him a favor, because he may have actually loved you--maybe he still does. Don't part in a way that could make him still long for you four years hence. Don't agree to disagree and amicably divide up the furniture on a Thursday evening. Kick the crap out of him. Make him loathe you. Fuck somebody else. Demonstrate no concern for his well being. Condescend. Criticize. Carp. Make sure that when you leave, he's glad you're gone. You probably owe him that much.