Ok, so you were a sinner before you were saved.
GET OVER IT!
Some people are not bold to serve the Lord because they feel as if past sin has taken the wind out of their sails.
This is a tool of the devil for making you idle, lazy and timid in doing great things for God.
If you boldly sinned before conversion, you should twice as boldly serve after conversion
Remember, the Holy Spirit convicts us of sin to save us and put us into service; Satan accuses us to cripple us and defeat us.
WHO ARE YOU LISTENING TO?
Listen to William Gurnall's advice on how to be a Christian in Complete Armor:
His [the devil's] aim is to discredit not the sins but the saints. Here his chief tactic is to deliver his accusations as if they are an act of the Holy Spirit. He knows a charge from God’s cannon wounds deeply; therefore, when he accuses a conscientious Christian, he forges God’s name on the missile before he fires it. Suppose a child were conscious of gravely displeasing his father, and some spiteful person, to harass him, wrote and sent him a counterfeit letter full of harsh and threatening accusations, copying the father’s name at the bottom. The poor child, already painfully aware of his sins and not knowing the scheme, would be overcome with grief. Here is real heartache stemming from a false premise - just the kind of thing Satan relishes.
Satan is a clever investigator. He closely observes the relationship between you and God. Sooner or later he will catch you tardy in some duty or faulty in a service. He knows you are conscious of your shortcomings and that the Spirit of God will also show distaste for them. So he draws up a lengthy indictment, raking up all the aggravations he can think of, then serves this warrant on you as though sent from God.
Re-read the Scriptures:
And he shewed me Joshua the high priest standing before the angel of the Lord, and Satan standing at his right hand to resist him. And the Lord said unto Satan, The Lord rebuke thee, O Satan; even the Lord that hath chosen Jerusalem rebuke thee: is not this a brand plucked out of the fire? Now Joshua was clothed with filthy garments, and stood before the angel. And he answered and spake unto those that stood before him, saying, Take away the filthy garments from him. And unto him be said, Behold, I have caused thine iniquity to pass from thee, and I will clothe thee with change of raiment. And I said, Let them set a fair mitre upon his head. So they set a fair mitre upon his head, and clothed him with garments. And the angel of the Lord stood by.”