How was Jesus the Seed of David if Jesus is God who created Man (Adam), and David is the seed of Adam? How is Jesus the Messiah, the seed of David, if Jesus is God, and God created Man (Adam), and David is the seed of Adam? This is a never-ending circle, and can't physically be possible. We are told in the Old Testament that the Messiah will be the seed of David. This prophecy is fulfilled in the Messiah, Jesus Christ. Joseph was not the father of Jesus; God was. So if Jesus is God, he is his own Father? 

This is very confusing to me.

How has this apparent contradiction—between Jesus being the seed of David and Jesus being God—been resolved by Christian theologians and denominations that affirm the Incarnation: that Jesus is fully human and fully God?
"How is Jesus the Messiah, the seed of David"? Simple: Jesus is the human aspect of the offspring of David. Before that, Jesus was spirit.

Jesus created all things, including humanity, as one of the persons of the Godhead before He was flesh and blood. It was only after Mary gave birth to Him that Jesus became flesh and blood. So this is not a circle. 

To clarify your points: 


As the Son of God in spirit form, Jesus created all things (John 1:1-3). 
The Messianic prophecies foretold of Jesus the Son of God as coming in the flesh, as the offspring of a woman. So Jesus, the Word, became flesh (John 1:14).