Sarah's unchanging soul unites both worlds

ברש"י שני חיי שרה כולן שוין לטובה.

In Rashi: "the years of Sarah's life" — they were all equal in goodness.

Rashi notes that the Torah repeats "the years of Sarah's life" to teach that every one of her years was equally good.

עפ"י מ"ש במדרש למה שני חיי שרה באחרונה לומר לך שחביבין חייהם של צדיקים לפני הקב"ה בעוה"ז ובעוה"ב.

This is based on what the Midrash says: why is "the years of Sarah's life" stated again at the end? To teach you that the lives of tzaddikim are precious before HaKadosh Baruch Hu both in this world and in the World to Come.

The repeated phrase signals that a tzaddik's life is equally cherished by Hashem in both worlds.

ובאמת זה בחי' מדת ההשתוות לצדיקים הגדולים כאבות ואמהות שהנשמה שנשתלחה בעוה"ז לא קיבלה שום שינוי כמו שהיתה בעולם העליון כך היתה בעוה"ז.

And in truth this is the quality of hishtavus (constancy/equanimity) belonging to the great tzaddikim such as the Avos and Imahos: the neshamah that was sent into this world received no change whatsoever — exactly as it was in the upper world, so it remained in this world.

The Avos and Imahos possessed perfect inner constancy: descending into this world did not alter their soul one bit from how it was Above.

ועושית שליחות הקב"ה בשלימות.

And it carried out the shlichus (mission) of HaKadosh Baruch Hu in completeness.

Unchanged by the descent, such a soul fulfills Hashem's mission perfectly.

ז"ש כולן שוין לטובה.

This is the meaning of "they were all equal in goodness."

Sarah's years were "equal" because her soul remained constant and good throughout, never wavering.

החיות בעוה"ז והחיות שלעולם הבא.

The vitality in this world and the vitality of the World to Come.

For her, the life-force of this world and that of the next were one and the same unbroken stream.

וע"ז כתיב חי חי יודוך.

And concerning this it is written, "the living, the living, he shall thank You" (Yeshayah 38:19).

The doubled "living" alludes to these two unified levels of life.

חיי עולם וחיי שעה.

Eternal life and temporal life.

The two "livings" are the everlasting life of olam haba and the temporal life of this world — which in a tzaddik become one.

כמ"ש לעיל מזה וזה ההשתוות הוא מדריגה גדולה מאוד:

As stated above regarding this; and this hishtavus is a very great level.

This unbroken constancy between the soul's heavenly origin and its earthly life is an exalted spiritual madreigah.

Summary: The Torah's repetition that Sarah's years were "all equal in goodness" expresses the lofty quality of hishtavus possessed by the Avos and Imahos: their neshamah underwent no change in descending to this world, remaining exactly as it was Above. Thus their life in this world and their life in the next form one continuous, unwavering goodness — a very great spiritual level alluded to in "the living, the living, he shall thank You."