I'm starting to hate my wife?

Oh God, help me!! I'm 25yrs old and have been marred for the last 4 years. I married my high school sweetheart after she became pregnant with my son, not so much because I was madly in love but because i didn't want to become a statistic. As the years have gone by, our relationship has soured and we fight constantly. She will never admit to being wrong about anything, she argues things that don't even make sense. Wats wrong with her? Why has she changed so much? Everything we speak about becomes an argument. We fight and have meaningless after, I sleep with her because I want to hurt her I can't even explain it. Sometimes I sleep with her to humiliate her. And i ume she sleeps with me in order to make up. My son who is very attached to his mother always looks at me as though I am the one starting the fights. I don't know what to do, I don't want a divorce, not only can I not afford it but I feel it would hang over my head for the rest of my life. I also don't want my son growing up in a broken home. What do I do? I am sooo, unhappy!!! There is so much more wrong with this relationship it would take me many pages to fully explain all the details. What should I do?