Do you believe when the priest celebrate the mass is a christ himself who is administering the sacrament?
No, as we believe that there is only one Christ who died once
and for all. The celebration of the mass is merely a recreation of
Calvary in our present time. Contrary to some protestant beliefs,
we are not resacrificing Christ, but because God is a non-temporal
being the sacrifice of Calvary is ever-present.
But, more to the point of your question: No, the priest is not
"a christ", but rather what is called "in persona christi" meaning
"In the person of Christ".