Wouldnt the knowledge that Constantine did not actually give the catholic church the power that everyone was told completely invalidate the catholic churches authority? Even for Catholics?
No. And before we get into this, there were only Catholics when the Donation was found to be a forgery (well, and Byzantines, but they were already not great fans of the papacy).
First, the Donation is only a claim to temporal authority, something the papacy had effectively lost by the 15th century when the forgery was discovered. By this time the papacy only held the papal states, and most temporal rulers were not greatly concerned with papal authority. The papal states in turn were granted by Charlemagne, and therefore legally held regardless of the status of the Donation.
Second, the Church had existed for at least 600 years before the Donation was forged, in which time it was recognised to be the prima inter pares of the patriarchal sees, of which only Constantinople still survived when the Donation was shown to be a forgery (and not much longer thereafter).
So, not so big a deal. Those who cared were already at odds with the papacy, and those who supported the papacy wouldn't have cared.