Praise be to Allaah.  

Allaah has told us that he will forgive all sins for the one who repents to Him. He says (interpretation of the meaning): 

“Say: ‘O ‘Ibaadi (My slaves) who have transgressed against themselves (by committing evil deeds and sins)! Despair not of the Mercy of Allaah, verily, Allaah forgives all sins. Truly, He is Oft‑Forgiving, Most Merciful’”

[al-Zumar 39:53] 

This includes all sins, even shirk. Whoever repents, Allaah will accept his repentance. 

With regard repentance from shirk in particular and the acceptance thereof, Allaah says (interpretation of the meaning): 

“And those who invoke not any other ilaah (god) along with Allaah, nor kill such person as Allaah has forbidden, except for just cause, nor commit illegal sexual intercourse and whoever does this shall receive the punishment.

69. The torment will be doubled to him on the Day of Resurrection, and he will abide therein in disgrace;

70. Except those who repent and believe (in Islamic Monotheism), and do righteous deeds; for those, Allaah will change their sins into good deeds, and Allaah is Oft‑Forgiving, Most Merciful”

[al-Furqaan 25:68-70] 

Allaah mentions the kufr and shirk of the Christians, then calls them to repent. He says (interpretation of the meaning): 

“Surely, disbelievers are those who said: “Allaah is the third of the three (in a Trinity).” But there is no Ilaah (god) (none who has the right to be worshipped) but One Ilaah (God —Allaah). And if they cease not from what they say, verily, a painful torment will befall on the disbelievers among them.

74. Will they not turn with repentance to Allaah and ask His forgiveness? For Allaah is Oft‑Forgiving, Most Merciful”

[al-Maa'idah 5:73-74] 

No matter how great the sin, the forgiveness, generosity and kindness of Allaah is greater. 

All you have to do is to turn to Allaah, regret what you did and resolve not to do it again. Then receive the glad tidings of the bounty and mercy of Allaah, for Islam wipes out whatever sins came before it, as the Prophet (peace and blessings of Allaah be upon him) said to ‘Amr ibn al-‘Aas (may Allaah be pleased with him): “O ‘Amr, do you not know that Islam wipes out whatever came before it.” Narrated by Muslim, 121; Ahmad, 17861. 

And the Prophet (peace and blessings of Allaah be upon him) said: “The one who repents from sin is like one who did not sin.”  Narrated by al-Tirmidhi and classed as hasan by al-Albaani.  

If a person repents to Allaah, Allaah accepts his repentance and forgives him. Allaah says (interpretation of the meaning): 

“And He it is Who accepts repentance from His slaves, and forgives sins”

[al-Shoora 42:25] 

“And verily, I am indeed forgiving to him who repents, believes (in My Oneness, and associates none in worship with Me) and does righteous good deeds, and then remains constant in doing them (till his death)”

[Ta-Ha 20:82] 

We have to think well of our Lord and hope that He will accept our repentance, for Allaah says, “I am as My slave thinks I am.” A hadeeth qudsi narrated by al-Bukhaari, 7066; Muslim, 2675. According to Ahmad with a saheeh isnaad:  “I am as My slave thinks I am, so let him think whatever he wants.” 

With regard to strengthening your faith, this may be done in many ways, such as the following: 

1 – Remembering Allaah a great deal, reading His Book and sending a great deal of blessings upon His Prophet (peace and blessings of Allaah be upon him). 

2 – Regularly offering the obligatory prayers on time and offering a lot of naafil prayers, so that one may attain the love and support of Allaah as it says in the hadeeth: “Whosoever shows enmity to someone devoted to Me, I shall be at war with him. My servant draws not near to Me with anything more loved by Me than the religious duties I have enjoined upon him, and My servant continues to draw near to Me with supererogatory works so that I shall love him. When I love him I am his hearing with which he hears, his seeing with which he sees, his hand with which he strikes and his foot with which he walks. Were he to ask [something] of Me, I would surely give it to him, and were he to ask Me for refuge, I would surely grant him it.” Narrated by al-Bukhaari, 6137. 

3 – Keeping company with good people who will help you to worship and obey Allaah and discourage you from disobeying Him.  

4 – Reading biographies of righteous people among the scholars, ascetics (zaahids), devoted worshippers and those who repented. 

5 – Keeping away from everything that reminds you of sin and calls you to it. 

In general, faith is increased by doing acts of worship and keeping away from haraam things. 

We ask Allaah to help you and to accept your repentance, and to guide your heart. 

And Allaah knows best.