There is a hierarchy of law. In the US we have local ordinances, we have state laws, we have federal laws and the supreme law of the land used to be the US Constitution. Every law has to conform to the Constitution.
Likewise with God. The Mosaic Law is subordinate to the Law of life.
Rom_8:2 For the law of the Spirit of life in Christ Jesus hath made me free from the law of sin and death.
That life is the law of love which rules over the Mosaic law.
Also, when Jesus said, "If you love me, keep my commands" it is not necessarily a statement of duty. Rather it is a declaration that because we love Jesus we keep his commands, and his command is to love one another as he loved us.
Romans 7:21-25 So I find it to be a law that when I want to do right, evil lies close at hand. 22 For I delight in the law of God, in my inner being, 23 but I see in my members another law waging war against the law of my mind and making me captive to the law of sin that dwells in my members. 24 Wretched man that I am! Who will deliver me from this body of death? 25 Thanks be to God through Jesus Christ our Lord! So then, I myself serve the law of God with my mind, but with my flesh I serve the law of sin.
In Romans 7, Paul said that the Law is holy, righteous, and good, that he desired to to do what is good, and he delighted in obeying God's Law, and that he served God's Law with his mind, but that contrasted that with a law of sin that stirred up evil passions to bear fruit unto death, that held him captive, that caused him not to do the good that he wanted to do, and they he served with his flesh, so I see no grounds whatsoever to think that the Mosaic Law is the law of sin when he was clearly contrasting the two. According to Deuteronomy 30:15-20, the Mosaic Law brings life and a blessing for obedience, according to Proverbs 3:18, the Mosaic Law is a tree of life for all who take hold of he, and according to Matthew 19:17, Jesus said if we want to enter into life, then obey the commandments. According to Romans 6:14, sin will no longer have dominion over us for we are not under the law, but under grace, so he was speaking about us not being under the law of sin, where sin had dominion over us.
The higher law of life was speaking to those who were under the lower Mosaic law. He was relating to them according to what they knew. He was trying to explain that there was a higher law than the Mosaic law. This is why he said that if you look upon a woman with lust you have already committed adultery. He was showing them that the law of love is deeper than the outward Mosaic law.
When Jesus said that if we look upon a woman with lust we have already committed adultery, he was not sinning in violation of Deuteronomy 4:2 by giving a new or higher law, but rather he was was simply fulfilling the Law by teaching how to correctly understand and obey the 7th and 10th Commandments against adultery and coveting in our heart as they were originally intended to be understood.
Love is a choice. Just as obedience to the Mosaic law is a choice. Gentiles were never under the law. The book of Romans was written to Messianic Jews in Rome who knew the Mosaic law. There were some Gentile believers, of course, but most of the Christians during that time were still Jews. Note what he says in Chapter 7:
Rom 7:1 Know ye not, brethren, (for I speak to them that know the law,) how that the law hath dominion over a man as long as he liveth?
His brethren were fellow Israelis who knew the law.
Sin is defined as the transgression of the Mosaic Law and Christ set a sinless example for his followers to follow how to walk in obedience to it, so to say that Gentiles were never under the Law is to say that they have they are free to do what God revealed to be sin, that they have never needed to follow Christ, that they have never needed grace, and that they have never needed Christ to give himself to redeem them from all Lawlessness and to purify for himself a people of his own possession who are zealous for doing good works (Titus 2:14). It says that Jesus gave himself to redeem us from all Lawlessness, not to redeem us from the Law, but for those who nevertheless still hold that he gave himself to free Gentiles from the Law, it is inconsistent to hold that position and the position that Gentiles were never under the Law in the first place. If Gentiles were never under God's Law, then He had no grounds why which to judge the world with a flood. The book of Romans was written to address issues that came up when the Jews returned to Rome after Emperor Claudius had expulsed them, with Gentiles not wanting to come back under Jewish leadership, which they either did not heed or understand.