Maybe you are asking why should I do Tikkun? Does it really matter what I did?
The answer is that anyone that desires to connect to Hashem in a full extent should worry maybe there is something from the past that holds him back.
Of course, the main issue is where you are holding now in your life, but as much as we are trying to perfect ourself now and prepare our future we should have in mind also to do Tikkun for the past.
The proof of this is that the Arizal gave to his students to do many many tikkunim, including meditations, fast days, and more. The Ariza gave them these tikkunim for things they did in this lifetime and even in previous lifetimes. We see that even a sin that one may have done in a previous incarnation can hold you back from being fully connected with Hashem.
There is a special quality of these days that is brought down in the Shaar HaKavaanot of the Arizal. There it says that each day of the week between Rosh Hashana and Yom Kippur has the potential for full atonement for all the things that a person did on that day through his life.
For example, now when you are reading this post its …lets say… Friday. if one will do teshuva (how much more so if one will fast) he will do a tikkun for all of the Fridays of his whole life!