The message in the Quran is 'Do not be sad -- Never lose hope of Allah's mercy'. Never. I suggest saving this post for future reference... when you're feeling down in the dumps.
'Certainly no one despairs of Allah's Mercy, except the people who disbelieve. ' (Qur'an 12: 87)
'[...] When the Messengers gave up hope and thought that they were rejected (by their communities), Our help came to them...' (Qur'an 12: 110)
'And we delivered him from the distress. And thus we do deliver the believers.'
(Qur'an 21: 88)
'And you were harboring doubts about Allah. There, the believers were tried and shaken with a mighty shaking.' (Qur an 33: 10-11)
Do not grieve over the hurt that is inflicted upon you by others, and forgive those that have ill-treated you.
The price of jealousy and rancor is enormous; it is the price that the revengeful person pays in exchange for his malice towards others. He pays with his heart, flesh, and blood. His peace, his relaxation, and his happiness - these he forsakes because he desires the sweetness of revenge and because he resents others.
Jealousy and rancor are illnesses for which Allah has given the cure and remedy:
'[Those] who repress anger, and who pardon men...' (Qur'an 3: 134)
'Show forgiveness, enjoin what is good, and turn away from the foolish [i.e. don’t punish them].' (Qur'an 7. 199)
'Repel [the evil] with one which is better [i.e. Allah ordered the faithful believers to be patient at the time of anger, and to excuse those who treat them badly], then verily! He, bettiveen whom and you there was enmity [will become] as though he was a close friend.' (Qur'an 41: 34)
Do not grieve over that which has passed you by in life, for indeed you have been blessed with much.
Contemplate the many favors and gifts that Allah has bestowed upon you and be thankful to Him for them. Remind yourself of Allah's many blessings, for Him, the Almighty, said:
'And if you would count the graces of Allah, never could you be able to count them.' (Qur'an 16: 18)
'And [Allah] has completed and perfected His Graces upon you, [both] apparent [i.e. Islamic Monotheism, and the lawful pleasures of this world, including health, good looks, etc.] and hidden [i.e. One's Faith in Allah (of Islamic Monotheism) knowledge, wisdom, guidance for doing righteous deeds, and also the pleasures and delights of the Hereafter in Paradise, etc.].' (Qur'an 31: 20)
'And whatever of blessings and good things you have, it is from Allah. Then, when harm touches you, unto Him you cry aloud for help.' (Qur'an 16. 53)
Allah said, establishing His favors upon man:
'Have we not made for him a pair of eyes; and a tongue and a pair of lips? And shown him the two ways [good and evil]?' (Qur'an 90: 8-10)
Life, health, the faculties of hearing and seeing, two hands and two legs, water, air, food - these are some of the more visible blessings in this world, while the greatest of all blessings is that of Islam and correct guidance.
What would you say to someone who offered you large sums of money in return for your eyes, your ears, your legs, your hands_ or your heart? How great is your wealth in reality? By not being thankful, you do not render justice to Allah's countless favors.
So never lose hope of Allah's mercy. Ask yourself, what have you to be thankful for?
