There may be a few things that need clarification.
Did the priest actually direct you to correct and falsehoods written in past emails and posts?
Have your harmed others by these falsehoods? To what degree then is reparation required?
Will it harm others if you correct past falsehoods?
I imagine you have already determined to avoid writing and sharing untruths in the future.
Having repented and determined to be truthful in future, you have met the requirement for repentance and purpose of amendment of your practices. However it is also the case that some things are private and others have no right to the knowledge, in which case you aren’t obliged to share it.
Your priest is of course the person to give you your answer as to any action required regarding past untruths. One would complete reparation reasonably quickly, but you need to be sure that this is what is required in every case or if the priest would consider that a greater harm could be in telling everyone your untruths.
God grant you peace.