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.
Well that would be true if it were solely hermeneutics that were going into this. Remember, we don't hold to Sola Scriptura! :)
Look, we simply do not approach Scripture as if we were a purely blank cultural/historical slate, and as if the Holy Spirit would somehow be our infallible guide to reconstructing Christian worship based on what we find in the NT (or what we think we find).
For us the faith began as a community not as a book, with traditions passed down from the Apostles to each subsequent generation in various places and among various cultures. The book, i.e. the NT, is an infallible guide to Christianity, but it is not a complete guide to Christianity....as evidenced in the fact that the Trinity does not explicitly appear in it.
If you ask me, can I prove the Catholic priesthood from that text, of course the answer is no. I can't prove that anymore than you can prove Wednesday Night Bible Studies. But understanding that the priesthood was already there from Peter/James/John through Clement through Irenaeus all the way up to the present...then we can look at texts like that and see a clear relationship.