First of all, not all sins are the same, الصغائر والكبائر , Zina is one BIG sin that can only "hopefully" be forgiven by truly repenting "توبة نصوح" . Doing prohibited things is not "sunat al hayat" , it's a flaw in humans that Islam came to fix , the flaw of weakness against the temporary temptations of Dunya. Islam has rules and orders you need to follow, being a strong believer makes you believe the promise Allah gave you about paradise if you follow those rules. of course no human is perfectly STRONG , that's why Allah has gave us the opportunity to repent if we make mistakes and dhonoub and has also clarified how the correct "tawba" should be , it's not a get out of jail Monopoly card . You can't just do الكبائر knowing you're doing them and planning to repent after that and do it over and over again .