MM. Please advice whether this sinless perfect character that we must achieve, is a prerequisite to enter heaven and have eternal life? If reading sister White books and quotes you gave, it seems yes. Meanwhile, justification and eternal life is God free gift to humanity based on what Christ had done for us that we accept and made effective through faith. Thus, a Free gift with Christ as the focus of faith, versus a joint operation of labors between the Spirit and our will (grace + works) to attain sinless perfect character as the target, which is prerequisite to enter heaven and have eternal life.

Am I wrong?

Yes, a sinless, perfect character is a prerequisite to enter eternal life. Otherwise Adam wouldn't have been expelled from Eden.
And yes, if we still don't have a perfect character, we inherit eternal life through the perfect character of Christ, which covers ours. But if we have been saved, we will strive each day to live more and more in harmony with God's will, to have a character more and more like Christ's character. Otherwise we haven't been saved. This is what both the Bible and Ellen White say.

1Ts 4:3 For this is the will of God, your sanctification

Heb 12:14 Strive ... for the holiness without which no one will see the Lord.

Take these words in deeply and you will have your answers fulfilled, peace in the name of Jesus.

"Those who at Pentecost were endued with power from on high, were not thereby freed from further temptation and trial. As they witnessed for truth and righteousness they were repeatedly assailed by the enemy of all truth, who sought to rob them of their Christian experience. They were compelled to strive with all their God-given powers to reach the measure of the stature of men and women in Christ Jesus. Daily they prayed for fresh supplies of grace, that they might reach higher and still higher toward perfection. Under the Holy Spirit’s working even the weakest, by exercising faith in God, learned to improve their entrusted powers and to become sanctified, refined, and ennobled. As in humility they submitted to the molding influence of the Holy Spirit, they received of the fullness of the Godhead and were fashioned in the likeness of the divine. {AA 49.3}
The lapse of time has wrought no change in Christ’s parting promise to send the Holy Spirit as His representative. It is not because of any restriction on the part of God that the riches of His grace do not flow earthward to men. If the fulfillment of the promise is not seen as it might be, it is because the promise is not appreciated as it should be. If all were willing, all would be filled with the Spirit. Wherever the need of the Holy Spirit is a matter little thought of, there is seen spiritual drought, spiritual darkness, spiritual declension and death. Whenever minor matters occupy the attention, the divine power which is necessary for the growth and prosperity of the church, and which would bring all other blessings in its train, is lacking, though offered in infinite plenitude. {AA 50.1}
Since this is the means by which we are to receive power, why do we not hunger and thirst for the gift of the Spirit? Why do we not talk of it, pray for it, and preach concerning it? (JSOT; Especially since He is lacking in so many) The Lord is more willing to give the Holy Spirit to those who serve Him than parents are to give good gifts to their children. For the daily baptism of the Spirit every worker should offer his petition to God. Companies of Christian workers should gather to ask for special help, for heavenly wisdom, that they may know how to plan and execute wisely. Especially should they pray that God will baptize His chosen ambassadors in mission fields with a rich measure of His Spirit. The presence of the Spirit with God’s workers will give the proclamation of truth a power that not all the honor or glory of the world could give. {AA 50.2}
With the consecrated worker for God, in whatever place he may be, the Holy Spirit abides. The words spoken to the disciples are spoken also to us. The Comforter is ours as well as theirs. The Spirit furnishes the strength that sustains striving, wrestling souls in every emergency, amidst the hatred of the world, and the realization of their own failures and mistakes. In sorrow and affliction, when the outlook seems dark and the future perplexing, and we feel helpless and alone,—these are the times when, in answer to the prayer of faith, the Holy Spirit brings comfort to the heart. {AA 51.1}
It is not a conclusive evidence that a man is a Christian because he manifests spiritual ecstasy under extraordinary circumstances. Holiness is not rapture: it is an entire surrender of the will to God; it is living by every word that proceeds from the mouth of God; it is doing the will of our heavenly Father; it is trusting God in trial, in darkness as well as in the light; it is walking by faith and not by sight; it is relying on God with unquestioning confidence, and resting in His love. {AA 51.2}
It is not essential for us to be able to define just what the Holy Spirit is. Christ tells us that the Spirit is the Comforter, “the Spirit of truth, which proceedeth from the Father.” It is plainly declared regarding the Holy Spirit that, in His work of guiding men into all truth, “He shall not speak of Himself.” John 15:26; 16:13. {AA 51.3}
The nature of the Holy Spirit is a mystery. Men cannot explain it, because the Lord has not revealed it to them. Men having fanciful views may bring together passages of Scripture and put a human construction on them, but the acceptance of these views will not strengthen the church. Regarding such mysteries, which are too deep for human understanding, silence is golden. {AA 52.1}
The office of the Holy Spirit is distinctly specified in the words of Christ: “When He is come, He will reprove the world of sin, and of righteousness, and of judgment.” John 16:8. It is the Holy Spirit that convicts of sin. If the sinner responds to the quickening influence of the Spirit, he will be brought to repentance and aroused to the importance of obeying the divine requirements. {AA 52.2}

We are saved by faith. We are saved based on what Jesus accomplishes for us. We cannot work our way to heaven or do anything to earn eternal life. We can, however, forfeit eternal life by refusing, neglecting, or failing to cooperate with the agencies of heaven to mature daily in the fruits of the Spirit. We are born again dead to sin, awake to righteousness, "complete in" (Col 2:10) Christ. "Perfect and complete in all the will of God" (Col 4:12).

Strive for holiness, yes, but not as a prerequisite for heaven, otherwise grace is no more grace.

He continued his strict and self-denying life, not now as the ground, but the result of faith; not the root, but the fruit of holiness. The grace of God in Christ is the foundation of the Christian's hope, and that grace will be manifested in obedience. Wesley's life was devoted to the preaching of the great truths which he had received--justification through faith in the atoning blood of Christ, and the renewing power of the Holy Spirit upon the heart, bringing forth fruit in a life conformed to the example of Christ. {GC 256.3, italics in the original, bolds supplied}

As long there is not a word about sinless perfect character as a prerequisite for justification to enter heaven and live eternally, I have no objection.

James, it's not a prerequisite for heaven. Ellen White never said that.