Happiness uninterrupted and complete

Cod. Bibl. Capit. Veron. LXXXV [80].
Replace with the very collect once available here.

Give to us, we pray, O Lord our God, always to rejoice in your devotion, because perpetual and complete is [our] felicity if we serve continually the author of all goods. Through.

"Da nobis, quaesumus, Domine Deus noster, in tua semper devotione gaudere, quia perpetua est et plena felicitas, si bonorum omnium iugiter serviamus auctori.  Per."

     Collect for the 33rd Sunday in Ordinary Time.
     This is no. 934 in Corpus orationum, which traces it back to Mohlberg no. 486 in the Veronese or "Leonine" sacramentary (Cod. Bibl. Capit. Veron. LXXXV [80]) written in the early 7th century, but containing prayers dating back into the early 5th (400-440 according to the ODCC).  (The scholars consulted through 1956 by Mohlberg had attributed this collect to Popes Damasus (366-384), Leo I (440-461), Gelasius I (492-496), and Vigilius (537-555), i.e. to periods ranging from the mid-4th to the mid-6th century.)  But it is also no. 2599 (sacramentaries from the 8th century and later):

"Fac nos, quaesumus, domine deus noster, in tua devotione gaudere, quia perpetua est et plena felicitas, si bonorum omnium serviamus auctori."

Cause us, we pray, O Lord our God, always to rejoice in your devotion, because perpetual and complete is [our] felicity if we serve continually the author of all goods.  Through.

The former is mistranslated in the 2010 Missal as:

"Grant us, we pray, O Lord our God, the constant gladness of being devoted to you, for it is full and lasting happiness to serve with constancy the author of all that is good.  Through."

And, according to Fr. Z, in the 1973 Missal as:

Father of all that is good, keep us faithful in serving you, for to serve you is our lasting joy.