The Godhead is three Persons, not one.
This. “One God in three Divine Persons” as the Baltimore Catechism puts it.
“[You] who live and reign forever and ever.” (And yes, qui vivis et regnas in saecula saeculorum is in the second person singular.) Not entirely wrong.
The prayers are addressed directly to God. We say “you live” and “you reign” not “you lives” and “you reigns”. Similarly, we say “you … who live” and “you … who reign”.
Maybe it sounds weird because the Latin-English Missal translations are “who liveth and reigneth…world without end”? (I still don’t know why the translation can’t be “ages of ages” as in the other languages. But “forever and ever”? Why the “and ever”? But I digress.)
In the third person, you would be correct. However, since the ends of the orations are in the second person (albeit usually with implied subjects, similar to the imperative mood, but actually being in the indicative mood), the conjugations do not end in s.
Most are that way, but not when the prayer is addressed to God the Son. The following prayer is part of every Mass:
In the way I was taught how to translation, “you” (or “thou”) is required in the English. “Who” is not specific enough and English is not inflective enough to go without a second person pronoun.“[You] who live and reign forever and ever.” (And yes, qui vivis et regnas in saecula saeculorum is in the second person singular.) Not entirely wrong.
That’s simple. Qui tecum vivit et regnat “who lives and reigns with you” here qualifies the preceding phrase “our Lord Jesus Christ” - so it would obviously be in the third-person. Here, however, qui vivis et regnas “who live and reign” (second-person) qualifies the addressee named at the beginning of the prayer (“Almighty and ever-living God”, etc.)