Hi, thank you for using health care magic. I can understand your concern. Masturbation as such does not produce any sexual dysfunction. It is normal practice in most of the male and no adverse effect have been proven. But it possible that your back pain may be related with positioning during masturbation, consult an orthopedic surgeon if it is producing significant distress. For premature ejaculation ,there is technique called start -stop technique , and squeeze technique which can offer you more control over ejaculation. this will reduce premature ejaculation over time. Medicine like sertraline , escitalopram are also useful in reducing premature ejaculation , this medicines are usally prescribed by psychiatrist for treatment of depression but for your condition they are also useful. So i would suggest you to consult psychiatrist for further help if needed. Psychiatrist can also help you out through behavioral therapy in your condition. I hope this will help you. Regards.