Wife is unresponsive for intimacy

My age is 28 and our marriage has almost completed 6 years, alhamdulillah. My wife is almost the same age as me and we have 2 children, alhamdulillah. We both, alhamdulillah, try to practice the Deen as much as possible. The problem is, from just a few months after our marriage, my wife began to show complete reluctance in responding physically, and gradually it became very severe. I, physically, desire her almost every day, but she at best responds once or twice a month, and that too unwillingly. In this situation, I am mentally in great pain and, unwillingly, I am falling into sin. Even after every tawbah I cannot stay away from it. I have also told my wife about the sins, and she feels upset temporarily but then goes back to the same way. Even when I show her the hadith about not responding to the husband, she feels hurt, makes an intention to change, but practically there is no benefit. My wife otherwise is very practicing and fulfills all other rights of mine as a husband, mashaAllah. But in this matter she is extremely negligent, and because of it my normal life is being disrupted. In this case, please explain the ruling of Shariah.