Jesus is the third person of the Holy Trinity. The Holy Trinity, another mystery, consists of God the Father, Jesus the Son and the Holy Spirit.
All three are God. But each one is a separate living being. The Father is not Jesus or the Holy Spirit. Jesus is not the Father or the Holy Spirit. And the Holy Spirit is not the Father or Jesus the Son. Yet each one is separately God.
I accept it is difficult to understand. But let us persevere and try to consider how Jesus is both fully God and fully man; then perhaps some clarity may appear.
Jesus did not start to exist when He was born on earth by the Virgin Mary. He always existed as a Spirit, totally independent from God, Whom we call the Father, and the Holy Spirit. Jesus was, and is, a Spirit and a living God that always existed.
When He was born on earth He took the form of a human being. He was made just like us, flesh, blood, bones and so on. He was fully human from that moment of His birth onwards. And He was fully God from the beginning of time. As long as God and the Holy Spirit existed, Jesus also existed as a Spirit and a God.
When He walked this earth, the body that we saw and recognised as Jesus was fully a human being. Just like us. But the Spirit, the soul, within that body was Jesus, who existed since the beginning of time.
When Jesus performed miracles it was the Divine Godship within Him, His Spirit, who performed the miracles which the human body was seen to perform.
His physical body, which was seen by the people of the time, was a human body like ours with the same feelings, and emotions that we too experience in life.
Let us look at this from a personal basis. We are spirits. Yes, you and me, and everyone else are spirits. We sometimes call this spirit our soul. Although this is not strictly accurate. It is not our soul as such because more accurately we are a soul. It would be more accurate to say that we are a soul, a spirit, and we have a body within which we inhabit.
In a way, we are no different to Jesus in that respect. We are a soul, a spirit, within a human body.
The very important difference is that we are not a living God, and our spirit has not always existed. It started to exist when we are created by God at conception. And it will continue to exist when our human body dies.
Our spirit, or soul, does not die when our physical body dies and disintegrates. Our spirit/soul continues to live on for eternity.
When we hurt our body, say we hurt our finger or hand, it is the human body, the flesh and bones etc ... which feels the pain and reacts to it.
But when we act in life, when we decide what to do or say, or how we behave, it is us as a soul who decides how to behave. Our bodies may take the actions required, but it is us the "soul/spirit" who controls the action.
That is how Jesus was fully God and fully human. His Spirit was/is an ever existing Godly soul, and His body was fully human. Suffering the pains and agonies of torture and beatings when He died on the Cross. He felt every pain that was inflicted on Him just like any human would have done. Yet, His Divine Spirit accepted it all for our sake.
A mystery we will never fully understand in this life.