Jesus said that He was with His Father before the world was created.
Joh 17:5 KJV And now, O Father, glorify thou me with thine own self with the glory which I had with thee before the world was.
You are speaking falsehoods.
Does he say that he was with the Father?
No, not really.
The glory that I had with thee was with the Father, not Jesus himself.
If Jesus was literally with the Father then Matthew 1:18 is error.
Now the birth of Jesus Christ was on this wise: When as his mother Mary was espoused to Joseph, before they came together, she was found with child of the Holy Ghost.
The Greek word translated birth is gennessis, or beginning. Jesus had a beginning at it began when Mary was found with child of the Holy Ghost.
God certainly planned ahead for Jesus Christ. Genesis 3:15 was not an impulsive decision, God had prepared for man's redemption from the fall in the very design of the heaven and the earth.
God was well prepared and had things in motion immediately, including the glory that He had prepared for the eventuality of His son.
The statement is not difficult to understand. Even earthly parents are fully capable of preparing for children they do not yet have, yet plan to have.
They might choose a dwelling fit for raising children, they prepare a room for the child, diapers, a crib, even decorate the room so as to give the child every advantage that the parents can. God is no different. God not only foreknew His son, Jesus Christ but as as well and prepared for us well ahead of time
Ephesians 1:3-4
3 Blessed be the God and Father of our Lord Jesus Christ, who hath blessed us with all spiritual blessings in heavenly places in Christ:
4 According as he hath chosen us in him before the foundation of the world, that we should be holy and without blame before him in love:
Those spiritual blessings are the glory that we had with God before the foundation of the world.
Romans 8:29-30
29 For whom he did foreknow, he also did predestinate to be conformed to the image of his Son, that he might be the firstborn among many brethren.
30 Moreover whom he did predestinate, them he also called: and whom he called, them he also justified: and whom he justified, them he also glorified.
Just like Jesus Christ God had prepared for us as well.
May we learn to live in a manner commensurate with God's love to us.