Yes, repentance is absolutely necessary for salvation. No man is saved, unless he repents (turns; changes his mind).
When the gospel is preached (Jesus died for our sins, was buried, and rose from the dead) we turn (change our mind) about us making things right (righteousness by what we do) to God has made things right (righteousness by faith in what God has done through Jesus Christ).
We are not repenting from sins. The apostle Paul makes it clear that he found no hope in his efforts to repent from his sins...
"For what I am doing, I do not understand. For what I will to do, that I do not practice; but what I hate, that I do. If, then, I do what I will not to do, I agree with the law that it is good. But now, it is no longer I who do it, but sin that dwells in me. For I know that in me (that is, in my flesh) nothing good dwells; for to will is present with me, but how to perform what is good I do not find. For the good that I will to do, I do not do; but the evil I will not to do, that I practice. Now if I do what I will not to do, it is no longer I who do it, but sin that dwells in me. I find then a law, that evil is present with me, the one who wills to do good. For I delight in the law of God according to the inward man. But I see another law in my members, warring against the law of my mind, and bringing me into captivity to the law of sin which is in my members. O wretched man that I am! Who will deliver me from this body of death?"
"You have not yet resisted to bloodshed, striving against sin."
Paul knows full well that he has no ability to do anything about sin, so he repents (turns to God) who has sent His Son to die for our sins. No man has the ability to "repent from their sins", and must turn to the only One who is qualified to take away our sins.
"There is therefore now no condemnation to those who are in Christ Jesus, who do not walk according to the flesh, but according to the Spirit."
This is where sound doctrine takes a "turn", and why you have been taught, and are teaching others to "repent from their sins". False teachers claim that "walking according to the flesh" is the same as "sinful behavior", but that is contrary to everything Paul just said in the preceding verses. Walking according to the flesh, is to serve the Law, which would return Paul to condemnation, for through the Law, comes the knowledge of sin.
"I would not have known sin except through the law. For I would not have known covetousness unless the law had said, “You shall not covet.” But sin, taking opportunity by the commandment, produced in me all manner of evil desire. For apart from the law sin was dead."
"For what the Law could not do, weak as it was through the flesh, God did: sending His own Son in the likeness of sinful flesh and as an offering for sin, He condemned sin in the flesh, so that the requirement of the Law might be fulfilled in us, who do not walk according to the flesh but according to the Spirit."
In Gal 3, Paul revisits this:
"For if a law had been given which was able to impart life, then righteousness would indeed have been based on law. But the Scripture has shut up everyone under sin, so that the promise by faith in Jesus Christ might be given to those who believe."
When Paul was in the flesh, he waged war against the flesh, by observing his behavior under the Law, which proved him to be a slave of sin. He set his mind on the flesh, but now, having been circumcised from the flesh, he sets his mind on the things of the Spirit, where there is no more condemnation, where he can rest in the assurance that Jesus has dealt with sin, once and for all.
"For those who are according to the flesh set their minds on the things of the flesh, but those who are according to the Spirit, the things of the Spirit. For the mind set on the flesh is death, but the mind set on the Spirit is life and peace, because the mind set on the flesh is hostile toward God; for it does not subject itself to the Law of God, for it is not even able to do so, and those who are in the flesh cannot please God."
The slave of sin, whose mind is set on the flesh, is hostile toward God. He sees the Law, but he has no ability to obey it. It's impossible for those who submit to it's demands to please God. 'Without faith it is impossible to please Him (Heb 11:6). The Law was fulfilled in us, who no longer walk according to flesh, but according to the Spirit (vs 4).
"However, you are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you. But if anyone does not have the Spirit of Christ, he does not belong to Him. If Christ is in you, though the body is dead because of sin, yet the spirit is alive because of righteousness. But if the Spirit of Him who raised Jesus from the dead dwells in you, He who raised Christ Jesus from the dead will also give life to your mortal bodies through His Spirit who dwells in you."
In Christ, the Spirit of God dwells in us. He is our life ("It is the Spirit that gives life" Jn 6:63) and we are the righteousness of God in Him (2 Cor 5:21). Because of the indwelling sin, the body is dead, and it will eventually be redeemed (Rom 8:23), but who we are now, as a new creation, is alive, because of the Spirit dwelling in us. We are "one spirit with Him" 1 Cor 6:17
Therefore, we are no longer to live in that system of sin and death, and continue to wage war against the flesh, by serving the Law, which can only bring about death ("The wages of sin is death"), but now we live by the Spirit and put to death (kill) that system of being right with God through the deeds of the body. We now walk, just as we received Him, by grace through faith in the finished work of the cross.
The old system returns us to being slaves of sin, which produces fear, but we are now slaves of righteousness, and God's children.
"So then, brethren, we are under obligation, not to the flesh, to live according to the flesh— for if you are living according to the flesh, you must die; but if by the Spirit you are putting to death the deeds of the body, you will live. For all who are being led by the Spirit of God, these are sons of God. For you have not received a spirit of slavery leading to fear again, but you have received a spirit of adoption as sons by which we cry out, “Abba! Father!” Rom 8:12-15