Of course, this prayer is Gregory writing after the fact, and so it's likely to have been embellished somewhat, given his great respect for his sister, and that grief always colours our recollection of events (either for good or for bad). Still, it's a beautifully poetic piece of writing, laden with Biblical imagery and theological significance and yet it remains strikingly timeless.
I hope you enjoy it and find benefit in praying along with a great woman of God... even if it is 1600 years after it was originally uttered.
She said: 'O Lord, You have freed us from the fear of death; You have made the end of life here the beginning of a true life for us. For a time, You give rest to our bodies in sleep and You awaken us again with the last trumpet. The dust from which You fashioned us with Your hands You give back to the dust of the earth for safekeeping, and You who have relinquished it will recall it after reshaping with incorruptibility and grace our mortal and graceless substance. You redeemed us from the curse and from sin, having taken both upon Yourself; You crushed the heads of the serpent who had seized us with his jaws in the abyss of disobedience. Breaking down the gates of hell and overcoming the one who had the empire of death, You opened up for us a path to the resurrection. For those who fear You, You gave as a token the sign of the holy cross for the destruction of the Adversary and the salvation of our life. O God everlasting, towards whom I have directed myself from my mother's womb, whom my soul has loved with all its strength, to whom I have dedicated my body and my soul from my infancy up to now, prepare for me a shining angel to lead me to the place of refreshment where is the water of relaxation near the bosom of the holy Fathers. You who broke the flaming sword and compassionately gave Paradise back to the man crucified with You, remember me also in Your kingdom, for I, too, have been crucified with You, having nailed my flesh through fear of You and having feared your judgments. Let the terrible abyss not separate me from Your chosen ones; let the Slanderer not stand in my way or my sins be discovered before Your eyes if I have fallen and sinned in word or deed or thought because of the weakness of our nature. Do You who have power on earth to forgive sins forgive me so that I may be refreshed and may be found before You once I have put off my body, having no fault in the form of my soul, but blameless and spotless may my soul be taken into Your hands as an offering before Your face.'... When she had completed the thanksgiving and indicated that the [later nocturnal] prayer was over by making the sign of the cross, she breathed a deep breath and with the prayer her life came to an end."Gregory of Nyssa, "The Life of St Macrina." In Saint Gregory of Nyssa - Ascetical Works. (Washington: Catholic University of America Press, 1967, 163-191), 180-181.