The point that is being made is that one is allowed to add a personal prayer to any of the middle brachos in shmona esrai as long as it is appropriate to the theme of the bracha. Thus, the theme of rafa-ainu is healing a sick person and it would not be allowed to say a different type of bracha (such as for parnasa) at that point. However, the theme for shma koleinu is asking Hashem to answer our personal request. Thus, if one has a different problem, and is not sure wheich of the brachos is appropriate, or has thought of it after having passed the appropriate bracha, then one can add it in there. In fact, the reason that the yehi ratzon is written in the sidur is because we are not fluent in formulating such a personal prayer any more. One must write it out for onself beforehand in order not to stumble over the words if attempting to use ones own words.
As an example, if one has been attacked by Lashon Hara, it might be possible to come up with an appropriate Yehi Ratzon for V'lamalshinim or for resolution of a court case in ...
Since we are no longer expert in coming up with these prayers and determining which one it would belong in, we use shma koleinu and find appropriate prayers that those greater than us have written.
personal prayers
One may add a personal prayer into any one of the 19 blessings,
providing that the supplication is relevant to the topic of the
blessing. For example, in the blessing of "Rifa'einu" (Health and
Healing), one may specifically ask that someone be healed from their
sickness.39 The exception to this rule is the blessing "Shema Koleinu"
(Acceptance of Prayer), where one may ask for any needs.
The best place to say a personal supplication is "Elokai Nitzor,"
which is said after the 19th blessing.40 At this point, some also have
the custom to say a verse that contains their name, or that begins
with the first letter of their name and ends with the last letter of
their name.
Can I Add My Own Prayers to the Amidah?
For starters, you can insert a short personal prayer within each of
the middle 13 blessings of the Amidah. Such a prayer should fit the
theme of that specific blessing. For example, ask for the speedy
recovery of specific person within the blessing of Refa-enu (Heal us),
or pray for livelihood in the blessing of Barech Aleinu (G‑d grant
blessing).
Requests that are unrelated to any of these 13 blessings can be added
to the blessing of Shema Koleinu (Hear our voice) since its theme is a
general request for our needs. Your personal prayer should be added
before the blessing's concluding words: ki attah shome'a tefilat kol
peh... ("for You hear the words of every mouth...").
Another place for personal prayer is the end of the Amidah, before you
recite the second Yiheyu leratzon... (May our prayer be desirous to
You...). Source: Shulchan Aruch, Orech Chaim 119:1. Footnotes
1.
"The Men of the Great Assembly knew which transmissions of G‑dly
vitality are needed at each point in time, evening, morning and
afternoon. As the spirit of G–d was upon them, they were able to
determine just the names, titles, words and letters necessary ...
Therefore, these prayers are real prayers, meaning that they are all
measured precisely according to the need" (Rabbi Dov Ber of Mezritch,
Likutei Amarim).
See also Torat Menachem, 12 Tammuz, 5720, s'if 20; Likutei Sichot,
vol. 16, page 578, citing the Ibn Ezra on the verse (Ecclesiastes
5:1), "G–d is in heaven and you are on earth. Therefore, make your
words few." By Yisroel Cotla
