The Mystical Body always refers to the Mystical Body of Christ, not personal bodies, parts of the soul, etc.
The Mystical Body of Christ is His mystical body resulting from his Glorified Resurrected Body. He died on Good Friday and came back to life (Resurrected) on Easter Sunday. However He still retained His Body. Mary Magedaline couldn’t stop hugging Him, Thomas gave a manual inspection of His Wounds, Jesus ate and drank with His disciples. Also He can walk through walls and goes place to place instantly.
At the Last Supper Jesus instituted the Eucharist. At that moment He allowed bread and wine to become His Body and Blood, Soul and Divinity. This moment of the Last Supper is the same moment of His Sacrifice at Calvary, and is the same moment of all Masses ever sung now and till Time ends.
The Bread at Mass becomes Jesus’s (God’s) Body. We eat His Body. You are what you eat. Therefore you are One with Jesus/God. Lots of people eat His Body. They are all One with Jesus/God. People in the past have eaten His Body, people in the future will eat His Body. They are all one in Jesus/God. That is the Mystical Body of Christ.
**Trivia: **
The Holy Spirit is the Soul of the Mystical Body of Christ.
The Church is the Bride of Christ. (That is why a man leaves his father and mother and clings to his wife, and the two of them become one body. Gen 2: 24)
