The Lord has sworn and will not repent: “Thou art a priest FOREVER according to the order of Melchizedek.”
You are not unaware of Catholic/Orthodox/Anglo-Catholic teaching on the priesthood, I assume?
I am puzzled. How can you site a verse from the Book of Psalms, that refers to Jesus, and apply that to the Roman Catholic priesthood? That is what is sometimes known as hermeneutical gymnastics.