How is repentance understood in Lutheranism?

Christ talks a lot about repentance, and how we must repent and believe to be saved, he asks the woman caught in adultery to sin no more (John 8:11). But if we are unable to do any good as Paul says in Romans (Romans 7:18), how can we repent of anything? And also, can we truly say we live a life of "repentance" if we will sin for the rest of our lives, sort of "the old Adam still haunting us", when in contrast we should be as "dead to sin" (i.e. sin no more)?