:salam2:
Qalb is the general word for heart. It comes from the root which means something that turns around and about and upside down. It is the nature of hearts that they are constantly changing, this is the normal state of our hearts. When Allaah ta’ala refers to emaan and diseases of the heart, qalb is used.
Dua you may make;
"Our Lord! Let not our hearts deviate (from the truth) after You have guided us..." surah Ali Imran/8
"Our Lord! Pour out constancy (patience) on us and make our steps firm..." surah al-Baqarah/250
اللّهُم مُصَرّف القُلوب ، صَرِّف قُـُلوبَنا على طَـــَاعَــتِـــك
Allahumma musarrifal-qulubi, sarrif qulubana 'ala ta'atika
"Oh Allaah, the Turner of the hearts, turn our hearts to Thine Obedience."
Sahih Muslim, Chapter 3, Book 033, Number 6418
Pertaining to the above dua;
Abdullah b. Amr b. al-'As reported that he heard Allah's Messenger (salallahu 'alayhi wasallam) as saying: "Verily, the hearts of all the sons of Adam are between the two fingers out of the fingers of the Compassionate Lord as on heart. He turns that to any (direction) He likes. Then Allaah's Messenger (salallahu 'alayhi wasallam) said: 'O Allaah, the Turner of the hearts, turn our hearts to Thine Obedience.'"
It is said that a sin will seek other sin and sins causes our eeman to fluctuate. However, knowledge of the deen will increase one's fear of Allaah, may we always be aware that Allaah is always there.
No matter how lousy you feel, brother, you must always try to do dhikr the best you can. Remember that;
"Allah burdens not a person beyond his scope. He gets reward for that (good) which he has earned, and he is punished for that (evil which he has earned)..." surah al-Baqarah/286
Here for an article to understand more the increasing and decreasing eman.
In sha Allaah...
May the brothers and sisters here make dua for you. :tti_sister: