Into your hands I commend myself, my body and soul

At this point in THE SMALL CATECHISM, Luther offers some “Prayers for Daily Use.” Here are the prayers for the beginning and for the end of the day, calling on God for the whole course of daily life:

A MORNING PRAYER
In the morning, when you rise you shall make the sign of the holy cross, and you shall say:
In the name of the Father, and of the Son, and of the Holy Spirit. Amen.
Then, kneeling or standing, you shall say the Apostles’ Creed and the Lord’s Prayer. Then you may say this prayer:
I thank You, my heavenly Father, through Jesus Christ, Your dear Son, that You have kept me this night from all harm and danger; and I pray You to protect me this day also from sin and every evil, that all my doings and life may please You. For into Your hands I commend myself, my body and soul, and all things. Let Your holy angel be with me, that the wicked foe may have no power over me. Amen.

And then you should go with joy to your work, after a hymn, or the Ten Commandments, or whatever your devotion may suggest.
 
AN EVENING PRAYER
In the evening, when you go to bed, you shall make the sign of the holy cross, and you shall say:
In the name of the Father, and of the Son, and of the Holy Spirit. Amen.
Then, kneeling or standing, you shall say the Apostles’ Creed and the Lord’s Prayer. Then you may say this prayer:
I thank You, my heavenly Father, through Jesus Christ, Your dear Son, that You have graciously kept me this day; and I pray You to forgive me all my sins where I have done wrong, and graciously keep me this night. For into Your hands I commend myself, my body and soul, and all things. Let Your holy angel be with me, that the wicked foe may have no power over me. Amen.
And then lie down in peace, and sleep.

Do any of you have problems with the practice of praying prayers written by someone else? Can anyone explain why this is all right to do?