I am at a loss for words.
do you really think you have been perfect this year? that you have not done a single thing you shouldn’t have? come on, be honest. if you really do a cheshbon hanefesh I am sure you can come up with some feelings of guilt.
and if Yom Kippur was “made up”, how do you account for all the Jews including the non-religious who feel compelled to observe it in some way?
Mussaf on Yom Kippur describes the entire ritual that took place in the beis Hamikdash on Yom Kippur. what is your outlook on that? that its a nice fairy tale?
These are just some thoughts of mine on the subject. real question is, why do you doubt that Yom Kippur is in fact legit?