I think you missed some things in the Old Testament.
The "Shema" Verse
For most Jewish people, the "Shema" is the foundational verse for the concept of a monotheistic God. It is the very heart of Judaism, and serves to confirm, in the Jewish mind, that there cannot be anything other than one God. The Shema verse is found in Deuteronomy 6:4, and says:
"Hear, O Israel: The Lord our God is one Lord."
In Hebrew it reads, "Shema Yisrael Adonai Elohenu Adonai Echad." The word Shema is the first Hebrew word in the passage and means "hear." At first glance this seems to support the Jewish (and anti-Trinitarian) concept of a monotheistic God. However, a careful examination of Deuteronomy 6:4 actually establishes, rather than refutes, the plurality of God. In fact, the Shema verse actually presents one of the strongest arguments for the tri-unity of God in the entire Bible. Here's why. The last word of the Shema verse is echad, which is translated into English as the word "one." This is what is known as a compound-unity noun - that is to say, a noun which demonstrates oneness or unity, but at the same time contains two or more entities. A number of scriptural examples will help make the point.
Genesis 2:24, speaking of the union of Adam and Eve, reads: "For this reason a man will leave his father and mother and be united to his wife, and the two will become ONE flesh." Here, the Hebrew word that is used for "one" (one flesh) is the same word for "one" that is found in the Shema verse - echad. It clearly speaks of the unity of more than one person into a united, or singular, entity.
In Genesis 1:5, Moses used this same word when he described the first day of creation: "And God called the light day, and the darkness He called night. And the evening and the morning were the first day." This "one" day, or "first" day is the Hebrew word echad. The one day referred to was comprised of both light and darkness - evening and morning.
And still another example is found in Jeremiah 32, verses 38-39 which reads, "And they shall be my people, and I will be their God; And I will give THEM ONE HEART." Here, the many people are given "one" (echad) heart. A unity of two or more individuals into oneness
Jesus was prophesied as God long before the 1st century (Isaiah 9:6; Jeremiah 23:5-6; Psalm 45:6).
Jeremiah 23:5-6 - The Messiah Prophesied as God
Jeremiah 23:5-6 - The Messiah as God
And since Jesus is God, there is no conflict with the First Commandment.