The Mabi"t in his sefer Beis Elokim shaar Hatefilah perek 5 answered this question. He offers a m'halach from the Rashb"a as well as his own m'halach. The Rashb"a answered that while we have perfect faith in the Ribono Shel Olam and therefore we daven to him in first person i.e. atah chonen l'adam daas, still, as human beings with limited comprehension, we cannot possibly fathom His true m'tzius. Therefore we also add in elements of third person i.e. asher kidshanu who was m'kadesh us to recognize this fact.
The Mabi"t himself offers an alternate explanation which b'syat"a dishmaya also includes a concept related to inyana d'yoma namely, matan torah. While the Rashb"a was focusing on the two facets of Hashem, the Mabi"t focuses on the two facets of Klal Yisrael. He says, in fact, klal yisrael is not truly worthy of accepting the mitzvos Hashem. Indeed, if Adam Harishon - the y'tzir kapov of Hashem failed in the one mitzvah given him, how can we, a y'lud isha, possibly be worthy of acceptance of Hashem's mitzvos and Torah. Still, due to the chiba of Hashem to bnei Avrohom Yitzchok V'Yaakov He elevated us and purified us to enable us to be that vehicle of kabalos Ol Malchus Sh'mayim.
Says the Mabi"t, if you look into the parsha of matan Torah you will also find a discrepancy of the tenses. Sometimes it's in first person, like Atem r'isem, atem tihyu li, im sh'moya tishmi'u, while other times it's in third person, like v"chibsu simlosam, ki bayom hashlishi yeireid hashem l'einei kol ha'am. Why is this? To demonstrate that although we are now able to talk to Hashem in first person and do his mitzvos and follow His Torah, still, it was not because we were worthy of this but rather because Hashem elevated us to this status and to illustrate this we use a third person tense showing how removed we realy are from this level.
For this very reason concludes the Mabi"t certain brachos are also split into two tenses to demonstrate this point.
Hope this helps!
p.s. see mabi"t perek 20 for an alternate explanation (although quite similar to the Rashb"a above.
Good Yom Tov!