Why do I say "we stole" if I have never stolen?
The sources ask this themselves. Peninei Halakhah asks: how can someone who is careful with other people's money say "we robbed"? It is explaining the Yom Kippur confession, which uses these same words.
The first answer is the word "we." On Yom Kippur in the Temple, the Kohen Gadol (the High Priest) confessed for the whole people. Peninei Halakhah explains that after the Temple was destroyed, the Sages had each person say a general confession for all of Israel, while keeping his own wrongs in mind.
The second answer is that you can be part of a wrong without doing it. The Gemara says that if you could have protested what your family was doing and did not, you are held accountable for them. The same goes for your town, and for the world. Another passage in the Gemara says all Israel are responsible for one another, but it limits that: for most wrongs, it applies when people could have protested and did not (Shevuot 39a to 39b).
Sefer Chasidim (a book of ethics from Germany, around 1200) was talking about the Yom Kippur list of sins that carry the death penalty. Even a person who knows he did not do them must say them, because all Israel are responsible for each other. And a second reason: so the one who really is guilty is not embarrassed to confess. If everyone says it, no one stands out.
Peninei Halakhah adds that the confession clears even good people of their share, and wakes others to repent. These sources explain why the words are plural. They do not say the list describes you personally.
Peninei Halakhah, Days of Awe 7:4,Shabbat 54b,Shevuot 39a and 39b,Sefer Chasidim 601I say the same list every day. Is that real confession?
Fair question. The Rambam (Maimonides, 1100s) says what the Torah's confession is: you say what you did, that you regret it and are embarrassed, and that you will not do it again. "I did this and that." Whoever says more is praised. Ashamnu is not that on its own. It names kinds of wrongs, not what you did.
The law books know this. The Shulchan Aruch (the main code of Jewish law, 1500s) says there is no need to describe the sin, but you may, and if you confess quietly it is right to. The Rema (Rabbi Moshe Isserles, whose notes on it give the Ashkenazi custom) says the alphabetical Yom Kippur list, Al Chet, does not count as describing the sin, because everyone says the same words. It is prayer text.
So where does your own confession go? Rabbi Chaim Vital, who wrote down the Arizal's teachings, said he was not sure whether his teacher preferred a short "I sinned, I did wrong, I rebelled" or a detailed confession in the style of Ashamnu. He wrote that it was in doubt for him. He was speaking of a private confession inside the Amidah, in the blessing "Who hears prayer."
Peninei Halakhah gives the practical answer: when naming the wrong would help your repentance, naming it is better.
As for saying it again and again, the Rambam says you confess the same wrongs again each Yom Kippur, even if you have stayed on the right path. His proof is Tehillim 51:5, "my sin is always before me." That is once a year, not every day. But it shows that repeating is not a sign that the confession failed.
Rambam, Laws of Repentance 1:1 and 2:8,Shulchan Aruch, Orach Chaim 607:2,Sha'ar HaKavanot, the Amidah,Peninei Halakhah, Days of Awe 7:4Why confess my ancestors' sins when I did not do what they did?
The prayer says "we and our fathers have sinned." That is not a later invention. The Torah says that when Israel is in trouble, "they will confess their sin and the sin of their fathers" (Vayikra 26:40). And in Nehemiah 9:2 the people stood and confessed "their sins and the sins of their ancestors."
But the prophet Yechezkel (Ezekiel) says the opposite: "a son shall not carry the guilt of the father" (18:20). So how can we confess a father's sin?
Rabbeinu Yonah (a Spanish teacher of ethics, 1200s) writes in Sha'arei Teshuvah that a person should mention his own sins and his fathers' sins, because he is punished for his fathers' sins when he holds on to what they did. The Gemara faces the same tension with a verse in Devarim that says each person is put to death for his own sin. It explains that children are held accountable for their parents' sins only when they keep doing what their parents did (Sanhedrin 27b).
Peninei Halakhah, a modern law series, adds a practical angle. Along with honoring your parents, a person has to repair the bad habits that stuck to his family. By confessing them he shakes them off. If he does not, he may repeat them without noticing.
None of these sources asks you to feel guilty for something that happened before you were born. They tie your ancestors' wrongs to you only when you keep doing them.
Vayikra 26:40,Nehemiah 9:2,Yechezkel 18:20,Sha'arei Teshuvah 1:40,Sanhedrin 27b,Peninei Halakhah, Days of Awe 7:4