want children-do I leave or stay - I've been married 7 years to a woman 10 years my senior. She is unable to conceive on account of her age.

I've been married 7 years to a woman 10 years my senior. She is unable to conceive on account of her age. At the time of our marriage I was not interested in having children, but as I get older I find my urge to have a family grow stronger and stronger. I cannot afford a second wife (even if I could afford a second wife, my present wife would not permit that). I've suggested that she ask her sister in law to let us raise one of her younger children. My wife will not cooperate. As well, fertility treatments are not possible for my wife & would be too expensive. My only option is leaving my wife & marry my friend's sister. Am I committing a sin by leaving my wife for the sake of having children? I'm afraid that Allah will punish me for leaving my wife & causing her pain. However, If I stay with my wife I'll forever regret being childless. It hurts me to see friends & associates with children. I feel empty & alone without my own "flesh and blood" children. Being a "nice uncle" to other people's children is good enough. I want children that I can raise to be good Muslims & who will read Ya Sin for me when I die. I have no brothers, sisters, or other blood relatives to help me in my old age or to pray for me when I die. If I stay with my wife I will regret being childless. If I leave I'll feel guilty, I'll be hurting my wife, & fear that Allah will punish me. What should I do?