While you didn't explain your personal situation, there's a mistake people make that you may be making as well.
The Rambam says at the beginning of Hilchos Teshuvah (https://www.sefaria.org/Mishneh_Torah%2C_Repentance.1.1?lang=bi&with=all&lang2=en)
כָּל מִצְוֹת שֶׁבַּתּוֹרָה בֵּין עֲשֵׂה בֵּין לֹא תַּעֲשֶׂה אִם עָבַר
אָדָם עַל אַחַת מֵהֶן בֵּין בְּזָדוֹן בֵּין בִּשְׁגָגָה
כְּשֶׁיַּעֲשֶׂה תְּשׁוּבָה וְיָשׁוּב מֵחֶטְאוֹ חַיָּב לְהִתְוַדּוֹת
If a person transgresses any of the mitzvot of the Torah, whether a
positive command or a negative command [ital. mine] - whether willingly or
inadvertently - when he repents, and returns from his sin, he must
The Rambam goes on to explain how to do viduy, how to do teshuvah that allows the viduy. He talks about the obligation to do it before Yom Kippur, and how we are judged on Rosh Hashana.
Much later, starting in perek 5, the Rambam begins to speak at length about free will, our ability to change anything at all about ourselves, and finally - in perek 7 - our obligation to do teshuvah even on bad midos.
I think people mix these up. That second one is a long process (see his Hilchos Deios), perhaps a lifetime process, the process of becoming better and better. No one expects you to finish it before Yom Kippur.
Remember how the Rambam began: "If a person transgresses any of the mitzvot of the Torah, whether a positive command or a negative command..." That is what we are required to do each year, as soon as possible really. We are supposed to notice when we mess up badly, catch ourselves, and make sure it doesn't happen again.
We should not forget to do this, not let it get swamped by the overwhelming idea of fixing every single thing about ourselves that we should improve.
You mentioned, "take on a single small thing to improve on". That's for the second process, for building yourself up gradually. If someone had a serious collapse we don't tell them that. We tell them, Get up right away, get the help you need. You don't want to stay where you fell. Let's find ways to protect you from its happening again.
How often have you heard drashos that take for granted that right after Neilah, most people are going to go back to where they started? It's not supposed to be that way. We are supposed make sure that we are shomrei Torah and mitzvos, make sure that when we did mess up, we fix it and find ways to make it stay fixed. That's possible, because we aren't talking about an ingrained habit of our character, we're talking about something done wrong, carelessly, once. Or twice, ח"ו, but then it gets harder, so don't wait!
Or drashos about how very hard is it to have charatah today? Well, of course it's hard, if you're talking about bad midos that you haven't fixed yet and will need to continue working on for years. But charatah for something you are shocked that you ever did, that you never imagined you could have done? That charatah is natural.
I feel that this could make a big difference to people. If they do this, even if they have a million other things to fix, they are true בעלי תשובה and שומרי מצות. And they have fulfilled the mitzvah of teshuvah.
-Update: I was reading Pirkei Avos, "נדרים סייג לפרישות" - oaths are a [helpful] fence for building self-control. One way to understand this, maybe, is that nedarim are a tool to convert some part of the "working on oneself" process, which is fuzzy and slow, into issur v'heter, which is more clear and sharp-edged.