Al-jawab billahi at-taufeeq (the answer with Allah's guidance)
Do not lose hope in the Mercy of Allah Ta'ala and His forgiveness. Allah Ta'ala forgives all sins. If you are sincere in your repentance, verily Allah Ta'ala will forgive you, as He is Most forgiving, Most merciful.
Allah Ta'ala says : "Say: 'O My slaves who have transgressed against themselves (by committing evil deeds and sins! Despair not of the mercy of Allah; verily, Allah forgives all sins. Truly, He is Oft-Forgiving, Most Merciful'. (39:53)
"Pray to Him with fear and hope; His Mercy is near to the righteous." (7:56)
"Do not despair of the Comfort of Allah, none but unbelievers despair of the Comfort of Allah." (12:87)
"Will they not turn to Allah in repentance and ask His forgiveness? He is Forgiving, Merciful." (5:74)
"He who does evil or wrongs himself and then asks forgiveness of Allah will find that Allah is the Forgiver, the Most Merciful." (4:110)
"And those who, if they commit indecency or wrong themselves remember Allah and ask forgiveness of their sins - for who but Allah forgives sins.." (3:135)
Doubts and evil thoughts:
Doubts and evil thoughts are a sign of one’s faith.
Abu Haraira (RadhiyAllahu Anhu) narrates that once a group of people came to the Messenger of Allah (Sallallahu Alaihi Wasallam) and asked: “We experience such evil thoughts that it is impossible to bring them on our lips”. The Messenger of Allah (Sallallahu Alaihi Wasallam) said: “Do you really experience these thoughts? Yes, they replied. The Messenger of Allah (Sallallahu Alaihi Wasallam) said: “They are clear signs of faith”. (Muslim)
One should know that one is not accountable for the evil thoughts that occur in the mind and heart as long as they remain thoughts.
Abu Huraira (RadhiyAllahu Anhu) narrates that the Messenger of Allah (Sallallahu Alaihi Wasallam) said:
“Verily Allah forgives my Ummah for the evil thoughts that occur in their hearts until they don’t say it verbally or act upon it”. (Bukhari and Muslim).
Therefore, one does not come out of the fold of Islam due to experiencing these doubts and evil thoughts, neither is there any sin, as long as one remains a believer with his heart, mouth and action.
How to remove such thoughts:
Read the following Du'a from Quran (23:97-98):
رَّبِّ أَعُوذُ بِكَ مِنْ هَمَزَاتِ الشَّيَاطِينِ
وَأَعُوذُ بِكَ رَبِّ أَن يَحْضُرُونِ
Trans: And say: 'O my Lord, I seek refuge in You from evil suggestions of the satans. O my Lord, I seek refuge in You lest they attend me.'
Please read more: http://www.muftisays.com/qa/question/3803/urgent-help.htmlhttp://www.muftisays.com/qa.php?viewpage=viewQA&question=1850
Remember, those who believe in Allah Ta'ala and His Messenger (Sallallahu Alaihi Wasallam) and obeys them, there shall be No fear on them, nor shall they grieve i.e. they will enter Jannat (Paradise).
Allah Ta'ala says,
"We said: "Get down all of you from this place (the Paradise), then whenever there comes to you Guidance from Me (i.e. Quran and the Messenger (Sallallahu Alaihi Wasallam)), and whoever follows My Guidance (believes in me and obeys me), there shall be no fear on them, nor shall they grieve (in the Hereafter, they will enter Paradise)." (2:38)
To increase one's Imaan one must be obedient to Allah and His Messenger (Sallallahu Alaihi Wasallam) and perform righteous deeds, this will lead to Allah Ta'ala and Jannat.
Please view this link on ways to increase Imaan and to gain closeness to Allah Ta'ala: http://www.muftisays.com/qa/question/1779/guidance.html
Please view this link on Jannat and Jahannam:
And Only Allah Ta'ala Knows Best.