For we know that the law is spiritual: but I am carnal, sold under sin.  For that which I do, I allow not: for what I would (do), that do I not; but what I hate, that do I.  If then I do that which I would not (do), I consent unto the law that it is good.  Now then it is no more I that do it, but (it is) sin that dwelleth in me.  For I know that in me (that is, in my flesh,) dwelleth no good thing: for to will is present with me; but how to perform that which is good I find not.  For the good that I would I do not: but the evil which I would not, that I do.  Now if I do that I would not, it is no more I that do it, but (it is) sin that dwelleth in me.  I find then a law, that, when I would do good, evil is present with me.  For I delight in the law of God after the inward man:  But I see another law in my members, warring against the law of my mind, and bringing me into captivity to the law of sin which is in my members.  O wretched man that I am! who shall deliver me from the body of this death?  I thank God through Jesus Christ our Lord. So then with the mind I myself serve the law of God; but with the flesh (I myself serve) the law of sin.
It is ironic that the majority of our sins, as born again believers, are "secret sins." That is, they are sins others cannot see. How many time have you had evil thoughts? I cannot count the times hourly, let alone since I was saved! Hatred, lust, pride, unforgiveness, doubts, fears, and I am sure I am forgetting many of the sinful things that fill my mind. No one see them unless I allow them to motivate my words or my actions. But God sees them all! And the Word says they grieve the three members of the Godhead (Mk. 3:5; Eph. 4:30; Heb. 3:17).
I don't know about other believers, but I am terrible at confessing my sinful thoughts, and am not real good at confessing my less-than-Spirit-inspired speech. That is why I am especially glad God's Word says that if we confess our sins to God, He will forgive us of all our sins, and cleanse us of all our unrighteousness (1 Jn. 1:9). I take that to mean that if we confess the sins we realize we have committed, God forgives even those sins, of which we are unaware, for He says He returns His child to a state of righteousness!
The Cross paid for my sins, but fellowship with my Father requires constant confession.