When we believe the gospel and are baptised into Jesus Christ for the remission of past sins, we begin a new life which is to be lived by Christ’s commandments.
Romans 6:4 Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life.
If we are making a determined effort to live by Christ’s commandments, the sins we commit after baptism can be forgiven by confession in prayer.
1 John 1:7 But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin. 8 If we say that we have no sin, we deceive ourselves, and the truth is not in us. 9 If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness. 10 If we say that we have not sinned, we make him a liar, and his word is not in us.
It is natural and necessary to be aware of our sins, but the apostle Paul was there before us and he claims to be the chief sinner, so the title is taken.
1 Timothy 1:13 Who was before a blasphemer, and a persecutor, and injurious… 15 This is a faithful saying, and worthy of all acceptation, that Christ Jesus came into the world to save sinners; of whom I am chief.
How was Paul’s sin greater than ours? – Because he persecuted believers, hauling them off to prison and being party to the deaths of some.
Acts 26:9 I verily thought with myself, that I ought to do many things contrary to the name of Jesus of Nazareth. 10 Which thing I also did in Jerusalem: and many of the saints did I shut up in prison, having received authority from the chief priests; and when they were put to death, I gave my voice against them. 11 And I punished them oft in every synagogue, and compelled them to blaspheme; and being exceedingly mad against them, I persecuted them even unto strange cities.
I don’t think our sins come near to those of Paul, but my advice is that we should follow Paul’s example.
We can’t do anything about the past, so there is nothing to be gained by ruminating over it.
We can however change the future, and this is what Paul did.
Philippians 3:13 Brethren, I count not myself to have apprehended: but this one thing I do, forgetting those things which are behind, and reaching forth unto those things which are before, 14 I press toward the mark for the prize of the high calling of God in Christ Jesus.
Only the forgiveness of God is necessary for salvation.
Forgiving ourselves is irrelevant.
Hebrews 8:10 For this is the covenant that I will make with the house of Israel after those days, saith the Lord; I will put my laws into their mind, and write them in their hearts: and I will be to them a God, and they shall be to me a people: 11 And they shall not teach every man his neighbour, and every man his brother, saying, Know the Lord: for all shall know me, from the least to the greatest. 12 For I will be merciful to their unrighteousness, and their sins and their iniquities will I remember no more.
I hope you find this helpful.