the 955 pesukim of Devarim and the 955 gates of Heaven

רְא֣וּ עַתָּ֗ה כִּ֣י אֲנִ֤י אֲנִי֙ ה֔וּא וְאֵ֥ין אֱלֹקים עִמָּדִ֑י אֲנִ֧י אָמִ֣ית וַאֲחַיֶּ֗ה מָחַ֙צְתִּי֙ וַאֲנִ֣י אֶרְפָּ֔א וְאֵ֥ין מִיָּדִ֖י מַצִּֽיל

What exactly did Moshe see at this moment beyond the miracles of the 40 years in the desert that triggered this reaction?

R' Yosef Shaul Nathanson writes as follows:

ְThe Midrash in last week's parsha teaches that when Hashem told Moshe הן קרבו ימיך למות, Moshe responded and said, "How can you say that to me when I used that very word to proclaim הן לה׳ אלקיך השמים?" (Devarim 10:14) Hashem answered that this is the fate of all man from the time of the cheit of Adam, because הן האדם היה כאחד ממנו לדעת טוב ורע ועתה פן ישלח ידו ולקח גם מעץ החיים ואכל וחי לעלם.

What's the magic of the word הן that led Moshe to think that just because he used that word in connection to praising Hashem, Hashem therefore cannot use that word to tell him he is going to die ?

The AR"I has a teaching as follows: There are 955 levels in the heavens, corresponding to the gematriya of שמים (final letters continue the hundreds count after ת=400, so final chaf = 500, final mem = 600, etc). Of those 955, 900 have angels appointed over them, but the last 55 are filled with Hashem's presence alone and no angels may enter. This is the meaning of הן לה׳ אלקיך השמים, i.e. הן = 55 levels of the 955 = השמים belong to Hashem alone.

By coincidence (not), there are 955 pesukim in sefer Devarim. With every pasuk that Moshe recited, he opened another gate in heaven.

With every gate that Moshe opened, he tried to appeal the gezeirah against himself.

The malachim were all sworn not to help Moshe, and so for each of the first 900 gates that Moshe knocked on, so to speak, the malach in charge refused to carry Moshe's tefilos to Hashem and he was turned away.

When do you do when you get bounced from customer service agent to agent and can't get help? You ask for the manager, of course. Finally, after 900 pesukim, after being turned away by 900 angels, Moshe stood at the levels of heaven, the gates of prayer, that even angels had no power over -- only Hashem himself was there.  He could now take his appeal directly to the top.

Pasuk 900 is that pasuk of רְא֣וּ עַתָּ֗ה כִּ֣י אֲנִ֤י אֲנִי֙ ה֔וּא וְאֵ֥ין אֱלֹקים עִמָּדִ֑י אֲנִ֧י אָמִ֣ית וַאֲחַיֶּ֗ה מָחַ֙צְתִּי֙ וַאֲנִ֣י אֶרְפָּ֔א וְאֵ֥ין מִיָּדִ֖י מַצִּֽיל

Moshe Rabeinu now said to Hashem, b'shlama until now, it was angels turning me away, but I recognize that you Hashem have control over these 55 higher levels, הן לה׳ אלקיך השמים. Why then are You slamming the door in my face? Why even on these highest 55=הן levels do I deserve the same message of קרבו ימיך למות?

And the answer is that as great as Moshe is, הן האדם היה כאחד ממנו, no human can access those upper 55 levels lest man turn himself into a deity, as Chazal darshan כאחד ממנו refers to the "Yechido shel olam."