When the Yidden were in Galus Bavel, they married gentile women. When they came back to Eretz Yisroel, they were deeply ashamed, and they cried and cried. They divorced their wives, but they didn't know how to deal with such a horrific unbelievable Yerida (spiritual decline). This is Mamash the lowest of the low; to sell your Neshama in such a disgraceful way! How could they face Hashem? Despite the tremendous pain that Ezra had about this, he surprised them.
He said: לְכוּ אִכְלוּ מַשְׁמַנִּים וּשְׁתוּ מַמְתַּקִּים וְשִׁלְחוּ מָנוֹת לְאֵין נָכוֹן לוֹ כִּי קָדוֹשׁ הַיּוֹם לַאֲדֹנֵינוּ וְאַל תֵּעָצְבוּ כִּי חֶדְנַת ה' הִיא מָעֶזְכֶם נחמיה חי Go eat fat foods and drink sweet drinks! Being happy with Hashem is what needs to be. This remains a: הַלָכָה פְּסוּקָה applied Halacha, and we do just that during our Rosh Hashanah meals. Although every Yom Tov has to have Simcha, by having: בָּשָׂר וְיַיִן meat and wine and other forms of Simchas Yom Tov, on Rosh Hashanah there is a special Inyan to eat sweet foods, as a sign of having a good sweet year.
And perhaps the Inyan of being Osek in Teshuva has a special sweetness that we don't find elsewhere. Not only are we unconcerned about our past shameful history, but we move on as if nothing happened. We even act like we're in a sweet mode, and indeed the sweetest experience in the universe is a Teshuva Shleima, coming back to our beloved: אָבִינוּ שֶׁבַּשָׁמַיִם Father in Heaven.
These words of Chazal: בְּמָקוֹם שֶׁבַּעֲלֵי תְּשׁוּבָה עוֹמְדִים אֵין צַדִיקִים גְמוּרִים יְכוֹלִין לַעֲמוֹד ברכות לד ב "Baalei Teshuva are even GREATER than pure Tzaddikim!” need Chazara and Chazara (review again and again), because the Yetzer Hara is notoriously against your feeling good about your Teshuva. He'll plague you for the rest of your life no matter what. You still sinned! How can you live with yourself, after doing such a nasty thing??
Perhaps this is another reason why it's Assur (forbidden) to speak about sins on Rosh Hashanah. You need to get a loud and clear message that being Osek in Teshuva means the sweetest and happiest life, with loads of sugar & honey and delicious foods! This can also be a reason why first we have Rosh Hashanah to make us feel pure, & only then comes Yom Kippur when you address your Aveira issues.