You can find the “Prayer Before Reception of Holy Communion” by St. Thomas Aquinas here. The following prayer written by St. Thomas Aquinas is a prayer of thanksgiving to be recited after receiving the Most Precious Body and Blood of our Lord Jesus Christ.
I give Thee thanks, O holy Lord, Father almighty, eternal God, who hast vouchsafed, not for any merit of mine, but only out of the condescension of Thy mercy, to satisfy me a sinner; Thine unworthy servant, with the precious Body and Blood of Thy Son our Lord Jesus Christ.
I pray that this Holy Communion be not to me a condemnation unto punishment, but a saving plea unto forgiveness. May it be unto me the armour of faith and the shield of good will. May it be the emptying out of my vices, the extinction of all concupiscence and lust, the increase of charity and patience, of humility and obedience, and of all virtues; a strong defense against the snares of all enemies, visible and invisible; the perfect quieting of all my evil impulses, both fleshly and ghostly; a firm cleaving unto Thee, the one true God; and a pledge of a blessed destiny.
And I beseech Thee, that Thou wouldst vouchsafe to bring me, a sinner, to the ineffable banquet, where Thou, with Thy Son and the Holy Ghost, art to Thy saints true light, fullness of content, eternal joy, gladness without alloy and perfect bliss. Through the same Christ our Lord. Amen.