don't settle for a glass half full

 וָאֶתְחַנַּן אֶל⁠ ה׳ בָּעֵת הַהִוא לֵאמֹר  Why did Moshe pick that specific time --  בָּעֵת הַהִוא -- to daven and not earlier, when Hashem first told him that he would not enter Eretz Yisrael?  Rashi answers: בעת ההיא – לאחר שכבשתי ארץ סיחון ועוג דמיתי שמא הותר הנדר.  

There is a major machklokes Rishonim and Achronim as to whether mitzvos ha'teluyos ba'aretz apply in Eiver haYarden.  Some hold none apply mi'doraysa, only mi'derabbanan, others hold they do apply mi'doraysa, and some split between halachos like terumos u'maasros which do apply, and other halachos which do not.  Chazal darshen in a few places that certain halachos (orlah, nigei batim, korban ha'omer, shevi'is) apply in "HA'aretz" = Eretz Yisrael proper, THE land,  to the exclusion of Eiver ha'Yarden.  The question becomes whether these derashos are the exception that prove the rule that Eiver haYarden is otherwise the same as Eretz Yisrael, or do they establish that Eiver haYarden is in fact categorically different.  

Assuming that mi'doraysa mitzvos ha'teluyos ba'aretz apply in Eiver haYarden, and perhaps the war against Sichon and Og was a milchemes mitzvah of kibush ha'aretz, then Eiver ha'Yarden would seem to be, for all intents and purposes, the same as Eretz Yisrael.  Mishne laMelech in his sefer Parashas Derachim (derush #8) asks if so, what then does Rashi mean שמא הותר הנדר, maybe Hashem's oath was cancelled?  Were the neder in force, then Moshe should not only have been prevented from entering Eretz Yisrael proper, but should also have been barred from Eiver ha'Yarden as well!  

To make the same point in a slightly different way (see also the Meshech Chochma in this post ) , the gemara in Sotah (14a) writes:

דָּרַשׁ רַבִּי שִׂמְלַאי: מִפְּנֵי מָה נִתְאַוָּה מֹשֶׁה רַבֵּינוּ לִיכָּנֵס לְאֶרֶץ יִשְׂרָאֵל? וְכִי לֶאֱכוֹל מִפִּרְיָהּ הוּא צָרִיךְ?! אוֹ לִשְׂבּוֹעַ מִטּוּבָהּ הוּא צָרִיךְ?! אֶלָּא כָּךְ אָמַר מֹשֶׁה: הַרְבֵּה מִצְוֹת נִצְטַוּוּ יִשְׂרָאֵל, וְאֵין מִתְקַיְּימִין אֶלָּא בְּאֶרֶץ יִשְׂרָאֵל. אֶכָּנֵס אֲנִי לָאָרֶץ כְּדֵי שֶׁיִּתְקַיְּימוּ כּוּלָּן עַל יָדִי.

If mitzvos ha'teluyos ba'aretz apply in Eivar haYarden, then shouldn't Moshe have gotten what he wanted once he got to Eivar HaYarden?  

This question is a little less difficult, because there are mitzvos that apply only in Eretz Yisrael proper, e.g. you can't fulfill the mitzvah of binyan beis ha'mikdash in Eiver haYarden, and you can't conquer all of the 7 umos by staying in Eivar haYarden.  

To answer the Ml"M one is forced to say that even if mitzvos ha'teluyos ba'aretz apply in Eiver ha'Yarden, there is still a qualitative difference between those lands and Eretz Yisrael proper.  

Rav Shalom Messas, the former Sefardic Rav haRoshi of Yerushalayim, in his sefer derashot v'Yatazah haChamah offers a different explanation as to the significance of  בָּעֵת הַהִוא which I think is even more insightful when seen in context of the above discussion.  Before getting to his answer, let me preface one other question: why does Moshe bother to tell us about his tefilah to Hashem if at the end of the day it was rejected and made no difference?  

Rav Messas writes that the shevatim of Reuvain and Gad saw Eiver haYarden as being as good as, if not better than Eretz Yisrael proper.  This was their "eretz zavas chalav u'devash," as this was the land that could sustain their flocks.  Assuming that mitzvos ha'teluyos ba'aretz apply in Eivar haYarden, they had even more reason to stay put and not bother themselves with entering Eretz Yisrael proper.  They had all the upside of actually being in Eretz Yisrael without having to actually cross into the land.  Therefore, it was precisely at that moment, בָּעֵת הַהִוא, right after the conquest of Sichon and Og and the demand by Reuvain and Gad to settle in their territory, that Moshe chose to daven to be allowed to enter Eretz Yisrael and chose to share that tefilah with us.  Moshe is telling us not to make the mistake of thinking that a glass half full is satisfactory, that having Eivar haYarden fulfills the dream of yishuv ha'aretz and is enough to satisfy.  I had all that too, said Moshe, and I still poured my heart out to beg Hashem to let me enter Eretz Yisrael proper.  

In the famous address given by R' Tzvi Yehudah on Yom ha'Atzmaut 1967 (in full in Hebrew here and English translation here) he reflected on the joy with which the partition plan of 1947 was met in contrast to his own feeling of pain :

לפני י"ט שנה, באותו לילה מפורסם, בהגיע ארצה החלטתם החיובית של מושלי אומות-העולם לתקומת מדינת ישראל, כשכל העם נהר לחוצות לחוג ברבים את רגשי שמחתו לא יכולתי לצאת ולהצטרף לשמחה. ישבתי בדד ואדום כי נטל עלי. באותן שעות ראשונות לא יכולתי להשלים עם הנעשה, עם אותה בשורה נוראה, כי אכן נתקיים דבר ד' בנבואה בתרי-עשר - "ואת ארצי חילקו"! איפה חברון שלנו - אנחנו שוכחים את זה?! ואיפה שכם שלנו - אנחנו שוכחים את זה?! ואיפה יריחו שלנו - אנחנו שוכחים את זה?! ואיפה עבר הירדן שלנו?! איפה כל רגב ורגב? כל חלק וחלק, של ארבע אמות של ארץ ד'?! הבידינו לוותר על איזה מילימטר מהן? חלילה וחס ושלום!

באותו מצב מזועזע בכל גופי, פצוע כולי וחתוך לגזרים - לא יכולתי אז לשמוח. כך היה המצב לפני י"ט שנים, באותו לילה ובאותן שעות. למחרת בא אל ביתנו איש ברית קדשנו, הגאון רבי יעקב משה חרל"פ זצ"ל - היה לו צורך לבוא וכלום יתכן שלא היה בא?! התייחדנו אז שנינו, רגעים אחדים, באותו חדר קטן ומקודש שב"בית הרב" - ולאן יבוא אז אם לא לשם?! מזועזעים ישבנו ודמומים. לבסוף התאוששנו ואמרנו שנינו כאחד: "מאת ד' הייתה זאת, היא נפלאת בעינינו". נקבעה החותמת!

This is what Moshe was trying to tell us.  Sichon and Og were conquered, the people were now able to fulfill mitzvos that they were unable to fulfill all those 40 years in the desert, they now had land they could call their own to live in peace and raise their flocks -- you can imagine the people of Reuvain and Gad dancing in the streets, just like they danced in the streets in 1947!  Enter Moshe to throw cold water on the party and remind us to not lose sight of the bigger picture, not to be satisfied when there is so much more to strive for and achieve.  

But what are we supposed to do if that bigger picture is not politically achievable at this moment, or militarily achievable at this moment, or we are religiously not worthy of it at this moment?  What can we do?

Like Moshe, we can daven for it.  We can beg Hashem to allow us to get there.