When Paul declares "I am chief of sinners" he is testifying of sin that is most painful and troubling to him present , not past.
Were that true, a ton of Paul's other writings would be very hypocritical.
How can someone who doesn't believe a man can be sinless write to others praying for their perfection ?
For instance..."Finally, brethren, farewell. Be perfect, be of good comfort, be of one mind, live in peace; and the God of love and peace shall be with you." (2 Cor 13:11)
And..."Whom we preach, warning every man, and teaching every man in all wisdom; that we may present every man perfect in Christ Jesus:" (Col 1:28)
Or how can a sinner honestly write..."Awake to righteousness, and sin not; for some have not the knowledge of God: I speak this to your shame." (1 Cor 15:34)
Or..."Nevertheless the foundation of God standeth sure, having this seal, The Lord knoweth them that are his. And, let every one that nameth the name of Christ depart from iniquity." (2 Tim 2:19)
And with good reason.
Paul says that he was not even aware of his past sins prior to his salvation committed in ignorance.
1Ti 1:13
I obtained mercy, because I did it ignorantly in unbelief.
You seem to be unaware of what constitutes a sin.
James writes..."But every man is tempted, when he is drawn away of his own lust, and enticed.
15 Then when lust hath conceived, it bringeth forth sin: and sin, when it is finished, bringeth forth death." (James 1:14-15)
Without being tempted to commit a sin, there is no sin !
Without the lust to commit a sin, there is no sin !
God's biblical principle of unto whom much is given , much more is required applies to sin as well .
Where you aware of that ?
Don't you think that much was given to Paul ?
Don't you think that freedom from the lusts of the flesh, (Gal 5:24), would be one of those things ?
We can look to ourselves in this regard , what sin has been most painful and troubling for you in your life , sin you committed before you ever knew the Love of Christ , or sin that occurred after you had been GIVEN so much in Christ who died to be your Savior ?
With a true repentance from sin, there will be no more such pain or trouble.
That is insured by rebirth from God's seed, and the gift of the holy Ghost.
Which sin causes the most suffering and heartache, sin committed before salvation or after ?
Paul had an answer for this question.
Not the words of someone "boasting of their continued sinning" I might add.
Rom 7:24
O wretched man that I am! who shall deliver me from the body of this death?
This plight from Paul's remembrance of his pre-conversion failings under the Law was answered in Rom 6:6..."Knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin."
The body of sin/death of the pre-conversion Paul was destroyed at its baptism into Christ and into His death and burial.
For me the sin that is most painful and sorrowful & demoralizing is the sin that has occurred post-salvation, after I had received the greatest love, promise & assurance, any person can ever receive in the person of Jesus Christ.
One true repentance from sin would end your terrible pain and sorrow.
The sins I committed in ignorance before I ever received Christ , while not pleasant to think about will never compare to the heart felt pain from grieving the Holy Spirit in full knowledge of what Love was shown for me to save me.
Why would you fret over things you didn't even know were bad ?
You have been forgiven for your past, so why keep returning to them like they still mattered ?