Do not be sad: Seek forgiveness from Allah often, for your Lord is Of-Forgiving

I [Noah] said [to them]: `Ask forgiveness from your Lord; verily, He is Of-Forgiving. He will send rain to you in abundance; and give you increase in wealth and children; and bestow on you gardens; and bestow on you rivers. ‘
(Qur’an 71: 10-12)

So seek forgiveness from Allah more often and you will reap the benefits of doing so: peace of mind, lawful provisions, righteous offspring, and plentiful rain.

(And [commanding you]: Seek the forgiveness of your Lord, and turn to Him in repentance, that He may grant you good enjoyment, for a term appointed. And that He may bestow His abounding Grace to every owner of grace [i.e. the one who helps and serves the needy and deserving, physically and with his wealth, and even with good words]. (Qur ‘an 11: 3)

And the Prophet (Blessings and Peace be upon him) said:

“Whosoever seeks forgiveness (from Allah) often, and then Allah makes for him a good ending for every matter of concern and provides for him a way out of every tight situation.”

Related in Bukhari is a hadith that is known as the chief of Al-Istighfaar (i.e., of supplications with which one asks Allah for forgiveness):

“O’ Allah, You are my Lord and none has the right to be worshipped except you. You have created me and I am your slave; and I am upon your covenant and promise as much as I am able to be. I seek refuge in you from the evil that I have perpetrated. I confess to you your favor upon me, and I confess to you my sin. So forgive me; for verily, none forgives sins except you.”