Tawbah is not accepted from one who just says: “I'm sorry” There are a number of conditions that must be fulfilled in order for the Tawbah to be accepted - as mentioned by Imam Ali (A) in Nahjul Balagha Hadith no. 417:
Someone said before Amir al-mu’minin (A): "Astaghfiru'llah" (I ask Allah's forgiveness), then Amir al-mu’minin (A): Your mother may lose you! Do you know what "istighfar" (asking Allah's forgiveness) is? "Istighfar" is meant for people of a high position. It is a word that stands on six supports. The first is to repent over the past; the second is to make a firm determination never to revert to it; the third is to discharge all the rights of people so that you may meet Allah quite clean with nothing to account for; the fourth is to fulfill every obligation which you ignored (in the past) so that you may now do justice with it; the fifth is to aim at the flesh grown as a result of unlawful earning, so that you may melt it by grief (of repentance) till the skin touches the bone and a new flesh grows between them; and the sixth is to make the body taste the pain of obedience as you (previously) made it taste the sweetness of disobedience. On such an occasion you may say "astaghfiru'llah".