Question

what does You may have spent valuable, irreplaceable years trying to fit into the parade only to learn, too late, that you will never fit it.mean?

Answer 1

It means that a person has spent a lot of time and effort to fit into an organization or a group with a lot of enthusiasm and hope.  But he/she has learnt only after a significant amount of time that he/she would not fit into that group or organization.

So that person has to look for some thing else and his time and effort has been wasted.