Viduy
Rebbe Yaakov Yechezkiyahu of Pupa zt'l taught that before saying viduy, one should imagine himself walking alone in a forest and being assaulted by a band of bandits. They are sharpening their knives; he knows his end is near. He asks the thieves to allow him a final wish. His last wish is to say viduy. Imagine this scenario, and then say the viduy of Yom Kippur.
The Yismach Yisrael zt'l said that when he was young, he heard from magidim that when one says viduy, he should think: There will yet be another time when I will say viduy. That will be after my demise when I stand before the beis din in heaven. I will be dressed in a tallis and kittel, I will say viduy like I'm saying it now, but then it won't help. Now I can still fix everything with my viduy."
The viduy must be said with sincere regret and with a broken heart.
Rebbe Bunim of Peshischa zt'l taught that the primary judgment on Rosh Hashanah is to see whether we regret our aveiros.
Sometimes, it is very hard for a person to overcome his yetzer hara, and he performs an aveirah. But the question is, why didn't he regret what he did? He might be excused for the aveirah because the tests were great, etc., but he should at least regret what he did. What prevents him from regretting it? We say, ...לפניך שחטאנו חטא על, "For the sin that we committed before You..." Rebbe Asher of Stolin zt'l explained that our main regret is לפניך שחטאנו, that we sinned before You. People regret their aveiros for several reasons (fear of punishment, embarrassment, a feeling of failure and incompetence, etc.). Still, the ideal regret is לפניך שחטאנו, that we sinned before Hashem.
Nevertheless, since viduy is a mitzvah of the Torah, we understand that one must perform this mitzvah with joy. This might be the reason we say אשמנו and חטא על on Yom Kippur with a tune. We are happy because we know that with this viduy, we become close to Hashem.
