On this site there is an essay entitled “Weakness of Faith, which gives a detailed description of the weakness of faith that many people suffer, and explains the causes and the remedy. We advise you to read it. Here is the link:
There follow two pieces of advice from major scholars, in addition to the above, summing up what is required on your part:
1 – We advise you to read Qur’aan a great deal, listen to its recitation and ponder the meanings of what you read and listen to, as much as you can. If you find anything difficult to understand then ask the scholars in your country or write to other Sunni scholars for advice.
We also advise you to remember Allaah a great deal (dhikr) with the dhikrs narrated in the saheeh ahaadeeth, such as “Laa ilaaha ill-Allaah (there is no god except Allaah)”, “Subhaan-Allaah wa’l-hamdu Lillaah wa laa ilaaha ill-Allaah wa Allaahu akbar (Glory be to Allaah, praise be to Allaah, there is no god except Allaah and Allaah is Most Great)”, and so on. For more information on this see al-Kalim al-Tayyib by Ibn Taymiyah; al-Waabil al-Sayyib by Ibn al-Qayyim; Riyaadh al-Saaliheen [available in English]; al-Adhkaar al-Nawawiyyah by Imaam al-Nawawi, and so on.
Remembering Allaah (dhikr) increases faith and brings peace of mind. Allaah says (interpretation of the meaning):
“verily, in the remembrance of Allaah do hearts find rest”
Regularly perform acts of worship such as prayer, fasting and the other pillars of Islam, whilst hoping for the mercy of Allaah and putting your trust in Him in all your affairs. Allaah says (interpretation of the meaning):
“The believers are only those who, when Allaah is mentioned, feel a fear in their hearts and when His Verses (this Qur’aan) are recited unto them, they (i.e. the Verses) increase their Faith; and they put their trust in their Lord (Alone);
Who perform As-Salaah (Iqaamat‑as‑ Salaah) and spend out of that We have provided them.
It is they who are the believers in truth. For them are grades of dignity with their Lord, and forgiveness and a generous provision (Paradise).” [al-Anfaal 8:2]
Standing Committee for Issuing Fatwas, 3/185, 186
2 – Faith is also increased by obeying Allaah, and it decreases when one disobeys Him. So adhere to doing that which Allaah has enjoined such as performing the prayers on time in congregation in the mosque, praying zakaah willingly to purify yourself of sin and show compassion towards the poor and needy, sitting with good and righteous people so that they will help you to apply sharee’ah and guide you to that which will bring you happiness in this world and in the Hereafter.
Avoid the followers of bid’ah and sinners, lest they tempt you and weaken your resolve to do good.
Do a lot of voluntary good actions, turn to Allaah, and ask Him for strength.
If you do that, Allaah will increase you in faith and you will make up for lost time, and Allaah will increase you in goodness and steadfastness in adhering to Islam.
Standing Committee for Issuing Fatwas, 3/187.