Paul did not struggle with sin after Jesus saved him the way he did before Jesus saved him.
Paul in no way remained doing the sins that earned him the self-given title of Worst of Sinners. He said that about himself because BEFORE Jesus saved him, he was a violent man; he gave approval of Stephens’s death, guarded over the clothes that belonged to the men who killed Stephen. Saul put many of the saints in prison, and when they were put to death, he cast his vote against them. Saul began to destroy the church. Going from house to house, he dragged off men and women and put them in prison. Acts 22:4, Acts 22:20, Acts 26:10, Acts 8:3.
That is why Paul said he was the worst of sinners. Not because he kept sinning after he was saved!