The most sagacious of the sagacious ones is he who enters Paradise because of a sin he perpetrated: He places his sin before his eyes, continuing to be constantly afraid for himself on account of that sin until he departs from this world and enters Paradise. And the most foolish of the foolish ones is he who enters the Hellfire on account of a single good deed he performed: He places it before his eyes, constantly remembering it and hoping to be rewarded for it, and in the meanwhile, he takes his sins lightly; and he continues to remain upon that conditions until he parts from this world and enters the Hellfire.
My brother, be sagacious: be constantly afraid for yourself on account of your past mistakes, since you do not know whether or not your Lord will punish you for them. And you do not know what impact your sins will have on the remainder of your life.
Remember that Prophet Ibraheem (as), despite the fact that he was Khaleel Ar-Rahmaan (ie. position of high ranking with Allah), feared for himself and asked his Lord: “O my Lord…keep me and my sons away from worshiping idols.” (Qur’an 14:35)
And Yusuf (as) said: “My Lord…cause me to die as a Muslim (one submitting to Your Will), and join me with the righteous.” (Qur’an 12:101)
And Musa (as) said: “My Lord! For that with which You have Favored me, I will never more be a helper for the mujrimoon (criminals, polytheists, sinners)!” (Qur’an 28:17)
And Shu’aib (as) said: “And it is not for us to return to it unless Allah, our Lord, should will.” (Qur’an 7:89)
All of these Prophets of Allah feared for themselves (so imagine the degree t which we should fear for ourselves).
(Sufyan al Thawri)