Tawbah literally means to discontinue a sin and to return to the true path and in other words, to return to regretting the sin. [i]
Repentance is a move towards salvation and one's serious decision to stop committing sins is by itself a great victory which opens the gates of divine mercy to the penitent. In fact, he who has reached this stage (repentance) has gained a great victory and he must endeavor to strengthen his motive to combat against sins and to perpetually guard himself against them.
The important point to keep in mind is that one should not despair of divine mercy under any circumstances because the bigger the sin the greater would be the divine mercy. Allah, the Exalted, says:
"Say: O my servants! who have acted extravagantly against their own souls, do not despair of the mercy of Allah; surely Allah forgives the faults altogether; surely He is the Forgiving the Merciful [ii] . [iii]
What can be deduced from this and other Quranic verses and hadiths is that if the sinner becomes regretful of his/her sins and repents, Allah will accept his/her repentance and forgive them all, as if that person had never committed a sin to begin with. [iv] Based on this, it is clear that He will accept all of his past and future worships and good deeds. So never become hopeless of God’s mercy under any circumstances, because no matter how great the sin, Allah’s mercy is greater and all-encompassing.
For further information about repentance, refer to: Question 6148 (website: 6350).
[i] Farhange Mo’in, vol. 1, pg. 116.
[ii] Zumar:53 “قُلْ یا عِبادِیَ الَّذینَ أَسْرَفُوا عَلى أَنْفُسِهِمْ لا تَقْنَطُوا مِنْ رَحْمَةِ اللَّهِ إِنَّ اللَّهَ یَغْفِرُ الذُّنُوبَ جَمیعاً إِنَّهُ هُوَ الْغَفُورُ الرَّحیمُ”.
[iii] Adopted from Question 2130 (website: 2239).
[iv] “التائب من الذنب کمن لا ذنب له” (The repenter is like one who has never sinned). Kuleini, Kafi, vol. 2, pg. 435; Wasa’el al-Shia, vol. 16, pg. 74.