Berachos Daf 29
רבי אליעזר אומר כל העושה תפלתו קבע וכו׳: מאי ״קבע״? … רבה ורב יוסף דאמרי תרוייהו: כל שאינו יכול לחדש בה דבר.
We learned in the mishna that Rabbi Eliezer says: One whose prayer is fixed, his prayer is flawed. The Gemara asks: What is the meaning of fixed in this context?
Rabba and Rav Yosef both said: It refers to anyone unable to introduce a novel element, i.e., something personal reflecting his personal needs, to his prayer, and only recites the standard formula.
אמר רבי זירא: אנא יכילנא לחדושי בה מילתא, ומסתפינא דלמא מטרידנא.
Rabbi Zeira said: I could introduce a novel element in every prayer, but I am afraid that perhaps I will become confused. Consequently, there is no room to require the masses to introduce a novel element into their prayers.
What was the chiddush in the tefilla?
The Avnei Nezer (YD 454;24) explains based on Brachos 57 that it was a Tefilla to be cleansed of Chet. Rebbi Zeira, felt that there was always something that was preventing him from reaching Shleimut. Therefore, he always had what to add into his tefillos. This was akin to the Tefillos of Dovid HaMelech in Tehillim, always striving to grow and purify himself more. In order to have a flawless prayer, one needs to be able to recognize his own flaws. However, due to Rebbi Zeira’s righteousness, he was afraid he would become confused and off track. Therefore, Rebbi Zeira stopped adding these Tefillos.