The decision to live our lives for the sake of Allah is no guarantee that we will not forget Allah and commit mistakes for `all children of Adam are sinners, but best among sharers are those who constantly repent'. (Tirmidhi.)
Imam Bukhari recorded that even the Prophet, though he was sinless, `sought Allah's forgiveness 70 times a day'. We should therefore, constantly seek the forgiveness of Allah for every single sin, small or great.
Ask forgiveness from your Lord, then turn towards Him in repentance; He will loosen the sky over you in abundance, and He will add strength unto your strength. [Hud 11: 52.]
Seeking continuous forgiveness of Allah will assist in ensuring that our hearts and souls remain pure and wholesome. The Prophet said in this regard:
When a Believer sins, a black spot appears on his heart. But if he repents and seeks Allah's forgiveness, his heart becomes cleansed and polished. However if he continues to commit more and more sins [without seeking forgiveness, the spot continues to spread and ultimately covers all of his heart.
This is the rust which Allah mentions [in the Quran]:
What deeds they earn rust upon their hearts. [al-Mutaffifin 83: 14.] (Ahmad, Tirmidhi.)
Whatever the magnitude of our misdeeds, Allah is always ready to respond with an even greater amount of pardon. Therefore, never despair of the mercy of Allah, in seeking His forgiveness, even for those sins committed repeatedly: O My servants who have transgressed against themselves, despair not of Allah's mercy; for Allah forgives all sins, surely He is the all-forgiving, the Mercy-giving. [az-Zumar 39: 53.]
Thus, as long as there is hope there remains opportunity for redemption and salvation. Be aware, though, that the best form of repentance involves a firm resolve to reform and do better.
The Messenger of Allah said:
Remain conscious of Allah and Fear Him wherever you are, and follow a bad deed with a good deed which will wipe it out, and behave well towards people. (Tirmidhi.)