Vatican II on Priestly Improvisiation

Under general norms in Sacrosanctum Concilium:

22. 1. Regulation of the sacred liturgy depends solely on the authority of the Church, that is, on the Apostolic See and, as laws may determine, on the bishop.

2. In virtue of power conceded by the law, the regulation of the liturgy within certain defined limits belongs also to various kinds of competent territorial bodies of bishops legitimately established.

3. Therefore no other person, even if he be a priest, may add, remove, or change anything in the liturgy on his own authority.

The Priest cannot change the Liturgical Prayers because they are not his prayers. They are prayers of the Church, the prayers of the body of Christ, and so in a real way the prayers of Christ himself.