Occasionally a person faces various physical and spiritual sorrows and troubles in the world. Amongst these are feelings so intense that they cannot be compared with any other physical pain. This feeling that causes such great distress in the human soul is a feeling called "regret."
There are two completely different forms of regret, however. The regret felt by people of faith and the regret that non-believers experience. These two feelings are extremely different from one another.
Believers are the people who have an absolute faith in the fact that events take place by Allah's Will, and whatever befalls them does so by Allah's Will. This explains their all-important distinctive attribute of putting their unyielding trust in Allah, at good times, in trouble or when they make a mistake. The Prophet Muhammad (salAllahu alayhi wasalam) pointed out the strong character of the believer with a comparison in the hadith below:
The similitude of a believer is that of a standing crop in a field which is shaken by wind and then it comes to its original position but it stands at its roots.
[Muslim]
Upon making a mistake, a believer immediately repents with sincerity and hopes for Allah's forgiveness. As a result, he does not suffer from a distressful and long-lived feeling of regret. The regret felt by believers urges them to ask for repentance, to purify themselves and prevents them from repeating this error. It helps them rectify their errors and prevents them from plunging into a distressful and pessimistic mood. Moreover, this regret does not reduce their enthusiasm, devotion, or religious zeal nor does it drag them downward into a spiral of apprehension and depression.
Regret felt by disbelievers, on the other hand, is very distressing and long-lasting, as they do not put their trust in Allah when they encounter a difficulty or commit a transgression. Throughout their lives, they often use phrases like "I wish I had not done this..." "I wish I had never said this...," and so on.
More importantly, they are subject to a much greater regret in the Hereafter. Those who lived a life apart from the religion (deen) in this world will regret every misguided moment of their lives. They were warned before and invited to the straight path. They had enough time to contemplate and embrace the right way. Yet they did not listen when they were warned, ignoring the Hereafter as if they would never die. Then in the Hereafter, they will have no chance to get back to this world and correct their errors. In the Qur'an, Allah relates their regretful expressions as follows:
We have warned you of an imminent punishment on the Day when a man will see what he has done, and the disbeliever will say, "Oh, if only I were dust!"
[Surah an-Naba': 40]
If only you could see when they are standing before the Fire and saying, "Oh! If only we could be sent back again, we would not deny the Signs of our Lord and we would be among the believers."
[Surah al-An'am: 27]
They will say, "If only we had really listened and used our intellect, we would not have been Companions of the Blaze."
[Surah al-Mulk: 10]
I wish to warn people against a day when they will regret saying "had we only understood...," "had we only not rejected the signs of our Lord...," "had we only followed those who brought us the message...," "had we only done this and that" etc and to invite them to live for Allah while they still have the chance to correct their wrongs.
Keep in mind that that day no one's regret will save him from Allah's wrath. The only way to avoid this regret is to submit to Allah while there is still time and to comply with the commands of Allah.
Respond to your Lord before a Day comes from Allah which cannot be turned back. On that Day you will have no hiding-place and no means of denial.
[Surah ash-Shura: 47]