Where did the Jesus myth come from?

[FallentoReason]

Jesus was not a real person, so where did this Jesus myth come from?

Details about his life have been cherry-picked from the OT. Two examples to start you off:

Jesus being mistreated:

They spit on him, and took the staff and struck him on the head again and again. After they had mocked him, they took off the robe and put his own clothes on him. Then they led him away to crucify him.

I offered my back to those who beat me, my cheeks to those who pulled out my beard; I did not hide my face from mocking and spitting.

The words put into Jesus' mouth on the cross:

About the ninth hour Jesus cried out in a loud voice, "Eloi, Eloi, lama sabachthani?"—which means, "My God, my God, why have you forsaken me?"

My God, my God, why have you forsaken me?
Why are you so far from saving me,
so far from the words of my groaning?

Suspicious, I know.

Was it based on a single other faith?

Christianity isn't unique. Around the time of the supposed life of Jesus there were other mystery cults, such as Mithraism, who had identical beliefs to Christians today.

And this food is called among us Eucharistia, of which no one is allowed to partake but the man who believes that the things which we teach are true, and who has been washed with the washing that is for the remission of sins, and unto regeneration, and who is so living as Christ has enjoined. For not as common bread and common drink do we receive these; but in like manner as Jesus Christ our Saviour, having been made flesh by the Word of God, had both flesh and blood for our salvation, so likewise have we been taught that the food which is blessed by the prayer of His word, and from which our blood and flesh by transmutation are nourished, is the flesh and blood of that Jesus who was made flesh. For the apostles, in the memoirs composed by them, have thus delivered unto us what was enjoined upon them; that Jesus took bread, and when He had given thanks, said, "This do ye in remembrance of Me, this is My body"; and that, after the same manner, having taken the cup and given thanks, He said, "This is My blood"; and gave it to them alone. Which the wicked devils have imitated in the mysteries of Mithras, commanding the same thing to be done. For, that bread and a cup of water are placed with certain incantations in the mystic rites of one who is being initiated, you either know or can learn.

-First Apology; Justin Martyr, 150 CE

Another interesting thing about the early Christians is that there were 6 different schools of thought:

Marcionism – Christ was a purely spiritual entity
Nestorianism – Jesus and Christ were two different entities
Docetism – Jesus appeared physical, but he was really incorporeal
Apollinarism – Jesus had a human body and human soul, but a divine mind
Arianism- Jesus was the son of God, not God himself
Catholicism – Jesus was fully human and fully divine, both God and the son of God

EDIT:

The funny thing with this is that it makes it clear who Paul was referring to when he would mention the heretics. Paul, being someone who always referred to a spiritual Christ (his confirmed letters predate the Gospels) was telling us to ignore the literalists that thought Jesus was a person i.e. all of Christianity today.

How did everything get flipped around so badly?

Or was it completely invented out of nothing?

There's actually nothing unique about it, so it wasn't invented. Ideas like the 'Son of God' were already in the minds of Greeks for ~2400 years prior to Christ with figures such as Heracles, Son of Zeus. Other details such as the Virgin Mary, Christmas and Easter all come from Pagan tradition (which mostly deals with the sun and moon).