"Every sadness will gradually diminish except the sadness over sins."
– Hasan al Basri (rahimahullah)

Causes of ‘eeman decreasing

Shaykh Muhammad al-‘Uthaymin states: 

"As for the decrease of iman, it has causes:

Committing acts of disobedience, iman decreases according to the class of sin, its amount, the indifference the person has towards it and the strength or weakness of the incentive to perpetrate it.

As for its class and amount; the decrease of iman by major sins is greater than that by the minor sins. The decrement of iman by killing a soul unlawfully is greater than (illegally) devouring valued wealth. Its decrease by two acts of disobedience is greater than one act of disobedience and so on.

As for being indifferent to the sin; if an act of disobedience emanates from a heart that thinks little of who it is disobeying and has little fear of Him, then the resultant decrease will be greater than if it were to emanate from a heart that glorifies Allah and is very fearful of Him except that an act of disobedience inadvertently occurs from him.

Concerning the strength of the motive for committing the sin; if an act of disobedience emerges from a person whose incentives for doing the sin are weak, then the resultant decrease of his iman will be much greater than if it were to emerge from one whose incentives for doing the sin are strong.

This is why the display of arrogance by a poor person and an illegal sexual encounter by an old person are more sinful than the display of arrogance by a rich person and an illegal sexual encounter by a young person. As in the hadith: 'Three (types of people) whom Allah will not speak to. He will not look at them on the Day of Resurrection or sanctify them and they will have a painful punishment.'

Mentioned amongst these three are the gray-haired fornicator and the poor person who is arrogant, because of the lack of strong motives they have for embarking on these disobedient acts.” (Fath al Bariyyah pg. 65)

"يشفيك من كنت دوما بذكره تتغنى** والحال تحسن لما أحسنت بالله ظنا"
He (Allah) will grant shifaa (on the Last Day) to whoever would frequently mention Him, and your state will (only) improve when you have a good opinion (good thoughts) of Allah. 

"يَقول ابن القيم رحمه الله - الحُزنُ يُضعِفُ القَلب ، ويُوهن العَزم ويَضُر الإرادَة ، ولا شيءَ أحب إلى الشيطان مِن حُزن المؤمن
“Grief weakens the heart and determination, and hurts the will, and nothing is more beloved to Shaytaan than a sad believer”."
– Imaam Ibn al-Qayyim 

اللَّهُمَّ إِنِّي أَعُوذُ بِكَ مِنْ الْهَمِّ وَالْحُزْنِ وَالْعَجْزِ وَالْكَسَلِ , وَالْبُخْلِ وَالْجُبْنِ وَضَلَعِ الدَّيْنِ ,وَغَلَبَةِ الرِّجَالِ

pearlsofislam:

Allahumma inni a’udhu bika minal-hammi wal-Huzni wal-‘ajazi wal-kasli wal-bukhli wal-jubni wa dala’id-dayni waghalabatir-rijal.

[O Allaah, I take refuge in You from anxiety and sorrow, weakness and laziness, miserliness and cowardice, the burden of debts and from being over powered by men]

[Sahih Al-Bukhari, 7/158]