There is no doubt shirk is the most heinous of all sins in Islam; Allah says,

“Verily, Allah does not forgive (the sin of) of ascribing of divinity to anyone beside Him, although He forgives any lesser sin unto whomever He wills: for those who ascribe divinity to anyone beside Allah have indeed gone far astray.” (Qur’an: 4: 116)

The Prophet said, “Shall I tell you of the three most heinous sins? ...he then continued: (First and foremost of them is) shirk or associating partners with Allah….”

When Mu’ad asked the Prophet, peace be upon him, “How shall I get to jannah and ward of hellfire, he replied, “ You have indeed asked of a serious matter; and yet it is rather easy for those that Allah has made it easy: You shall worship no one but Allah and that you shall never associate any partner in worship with Him; establish prayers, offer zakah, fast the month of Ramadan, and make hajj to the sacred House---if you can afford to.”

Therefore, if you have been guilty of shirk in the past, the only option available to you is to enter Islam anew by saying the shahadah (the testimony of Oneness; ashahu an laa ilaaha illa Allah wa ashadu anna Muhammadan rasooluu Allah: I bear witness there is no god but Allah; I also bear witness that Muhammad is the Messenger of Allah). It is important also that you cherish you faith and seek to remain steadfast by consciously following the way of the Prophet, peace be upon him. You should never fail to turn to Allah to help you to remain steadfast. The following supplications can be read on a regular basis:

Allaahumma innee a’oodhu bika min an ushrika bika shay’an a’lamuhu wa astaghfiruka mimma a’alamuhu

(O Allah, I seek refuge in You from associating partners with You knowingly; I also ask forgiveness of You from associating any partner with You unknowingly).

Allaahumma yaa muqalliba al-quloobi thabbit qalbee alaa deenika

(O Allah, You are the twister/turner of hearts, make my heart firm on Your religion).