Rashi explains that they were all equally good. In definition that in every character and every situation it was good for her.
Kisvei R’ M. Midner – Sefer Toras Avos - Slonim
For I give you good doctrine, do not forsake my Torah. "כי לקח טוב נתתי לכם תורתי אל תעזובו" - אל תעזובו את המקח שנתתי לכם )מ"ר לג:א(
It is written: “I have given you good doctrine (לקח), do not forsake My Torah. Do not abandon the merchandise (מקח) which I have given you. - Medrash Rabbah (33:1)
One must think of the Torah as merchandise. A merchant never closes up shop because he has difficulty selling his merchandise, for he knows that once he closes up shop, he will certainly be ruined. The same is true of the Torah. Even if one has difficulty studying Torah, one must not stop - for once one ceases to study, one certainly will not be able to go on living.
So too, must we learn from the "Years" of Sarah, as Rashi explains שני חיי שרה - כלן שוין לטובה All are equally good.
We can go further with the Chidushei Harim's explanation of the Medrash;
Chidushei Harim on the Medrash
איתא במד "ר ר "ע הי 'יושב ודורש התחילו להתנמנם בקש לעוררן אמר מה ראתה אסתר למלך על קכ "ז מדינה .תבא בת בתה של שרה שחייתה קכ "ז שנים ותמלך על קכ "ז מדינות .וצריך להבין היאך בדברים הללו עוררן משנתן אלא משום שראינו ששרה חייתה קכ "ז שנה בעבור זה מלכה אסתר על קכ "ז מדינות ,א "כ החשבון עבור כל שנה מדינה אחת , ולפי החשבון יוכל להיות לכל שבוע עיר אחת ולכל שעה עכ"פ כפר אחד אשר שוויו כמה וכמה אלפים א"כ בעת אשר ישן אדם מעט יוכל להפסיד כמה אלפים משום זה נתעוררו משנתן :חידוש הרי "ם - ספר שיח שרפי קודש ח "א
R' Akiva was learning with his students, they started to slumber, He attempted to awaken them, He said how is it that Esther ruled on 127 countries? Let come the granddaughter of Sarah who lived 127 years and rule on 127 countries. We must now understand what is the power in these words to awaken the Talmidim [students] from their sleep? The answer is that we see that since Sarah lived 127 years [competent ones], Esther ruled over 127 countries, hence the computation is that for each year one country, and to this calculation it can be to each week, a city, and to each hour at least a town, which are worth many thousands [today many millions!]. If so in the time that a person slumbers a little he can lose allot of property, such a statement was a good reason for the Talmidim to awaken from their sleep.
Chidushei Harim - Sefer Siach Sarfei Kodesh
