This is a statement and a question in Is.43:19 that begins with "Behold". The "new" concerning our Creator is not new; but may appear to be new to us. We are required to master the basics, before moving on to the next grade. God[YHVH] first gives the information to His Priest and Prophets - to insure that the people are taught correctly.
Behold = lo, see, hey-hear-now, watch this, sit up straight with sharpen pencils. This word Behold is employed throughout Part 1 of the written Word. New = fresh, unused. Christ in Matt.26:28 "My Blood of the New Covenant[contract]". This new is in addition too: it does Not cancel or void previous covenants. Blood covenants were intended to last from generation, to generations.
This Holy blood is the foundation for many things: It remitted the sin from the Garden. It is the Passover covering that protects throughout time. It is the life for a New Nation of Christ Jesus: aka the Kingdom of Heaven, on earth-today.
Shall you not know it? This is a lack of faith, not paying attention, or too much time complaining. Jer.17:5 explains the curse involved in trusting flesh. 17:6 And shall not see when good comes. Premature death, or too busy complaining. They didn't think highly of the 'good-old-days' when they were in them.
The "New Thing" may be new to the world, but not to those following The Leader. Zach.13:9 is the process; and the result. Immanuel is real. The Kingdom of Heaven on earth-is now. How does one cover almost 7,000 yrs. in 7 seconds? Repent, for the Kingdom of Heaven - is here! Unused and fresh; "A New Thing".