Love and Marriage...not all it's cracked up to be.?

I was just wondering if there is anyone out there whom can tell me how a man can toss aside the woman who's been with him for 12 years for a skank? I've been with my husband for yes 12 years and have stood by him through horrible times. He had no car no job nothing when we met. I helped him financially get where he is today. I've been by his side through every storm...also, I am not unattractive. So how can he just without an ounce of care put another girl above me. She is young, very promiscuous, and has no job, but she does have a child. She is so trashy and wouldn't have a thing to do with him if he were where he was when I found him. How can he do this to me? Men? help me understand. Why doesn't my husband care about me the way he does for a girl he hardly knows and who doesn't care for him the way i do?