Yes, reading Harry's journal was a mistake. Dumbledore had decided this after reading the first sentence and had closed the book. It was one thing to be worried and protective, it was completely another to invade one's thoughts and experiences without consent.

Perhaps in time Harry would let him reading his journal if he proved he really was just concerned and really did want to repair his mistakes. Perhaps then Harry would allow him at least a glimpse into the journal but for now the journal was off-limits as was trying to control Harry in any way.

That had been one of the reasons why Dumbledore hadn't pushed Harry to change back. Commanding Harry was not something you just do like commanding a chess piece the only way to successfully have Harry on your side was to let him allow it to happen and give him the idea of exactly what you were going to do. That meant no more secrets. No more planning things without Harry's consent. And above all that meant informing Harry of every piece of information that he had even the most secretive and hidden plots would need to be laid out before Harry and Harry would need to decide whether to keep them or wipe out the plans entirely.