Is "You cannot love your neighbour unless you love yourself" a form of antinomianism?

Applying cultural context can aid in understanding scripture.

Antinomianism does not appear to enter into this; a cultural disconnect is more likely.

Antinomianism: ... from two Greek words, anti, meaning "against"; and nomos, meaning "law." Antinomianism means “against the law.” Theologically, antinomianism is the belief that there are no moral laws God expects Christians to obey.

Reading (and then restating in different terms) the scripture as described seems to contradict the teaching that was going on, as described by John J. Pilch.

Another custom was to sum up the Torah’s commandments in a small number of precepts or a summary statement. Thus King David proposed eleven (Ps 15), Isaiah six (33:15), Micah three (6:8), and Amos only one (5:4).

In reply to the Pharisee’s question about the “greatest commandment,” Jesus combines two: “Love the Lord your God with all your heart, and with all your soul, and with all your mind” (citing and amending Dt 6:5). And the second of equal importance is “love your neighbor as yourself” (citing Lv 19:18). Jesus does not discard other commandments. He explicitly adds: “On these two commandments hang all the law and the prophets.”

Pilch has written articles, and books, about the cultural context of first century life in the Middle East (which he often refers to as Mediterranean Culture). He presents various New Testament encounters couched in the cultural context of a given scene, interaction, or teaching. A useful caveat on reading scripture from "A Cultural Handbook to the Bible' explains:

The task of interpreting the Bible — which was written by and to people living in very different cultural contexts from contemporary Western society — can seem monumental. The opposite is also true: people can easily forget that studying the Bible is a type of cross-cultural encounter, instead reading their own cultural assumptions into biblical texts.

The passage you cite, rather than the variation offered, has underneath it an embedded cultural assumption of first century Mediterranean Culture, which is spelled out in Ephesians 5:29 by the Apostle Paul.

Paul was the Apostle to the Gentiles, an audience who were often not steeped in the base culture in which the New Testament narrative takes place.

In other words, anyone contemporary to Jesus who heard

"You shall love your neighbour as yourself,"

is already making the cultural assumption spelled out later in Ephesians 5:29 (KJV).

For no man ever yet hated his own flesh; but nourisheth and cherisheth it, even as the Lord the church:

Taking the teaching as Jesus presented it, and then changing it, seems to be willful misinterpretation of scripture rather than antinomianism.

You cannot love your neighbour unless you love yourself."

As Jesus said (roughly) to Pilate during the interview: your words, not mine.

Put differently, the embedded assumption is that you love yourself. It is a 20th century overwrite to throw doubt into that cultural assumption.

For fuller context on the Ephesians passage, Paul was speaking about marriage and love as a reflection of the Church as the body of Christ:

28 So ought men to love their wives as their own bodies. He that loveth his wife loveth himself.
29 For no man ever yet hated his own flesh; but nourisheth and cherisheth it, even as the Lord the church:
30 For we are members of his body, of his flesh, and of his bones.

One's wife is someone outside of himself, as is one's neighbor, so the embedded assumption that one does not hate one's self (while modern secular culture seems to accept self hate as normal (perhaps?) or common(?)) the message is clear and consistent with the commandment.

Applying the modern self-hate concept is where the confusion arises; that isn't scriptural, nor a matter of the law, but rather a "loss in translation" across the cultural divide. (And possibly scriptural distortion, depending upon how the translation from the Greek to English proceeded).