In my opinion you just needed a new instructor and better self-planning from yourself. The problem is you pick up some habits whilst driving on a full license, they're not necessarily bad they're just realistic, but some of them will get you marked down on a test. For example, on the road to my house between 7am and 7pm, the bays on the main road are in use which partially blocks the left lane, so I keep right all the way down to my road and then turn in just after the bays, whilst this is good planning from me because it lowers the queue of people filtering in from the left who don't know this - on a test I'd get screwed for not keeping left then turning into the right lane once I spot those cars. I do the same thing on the roundabout to work, I know I'm turning right shortly after the roundabout, so I keep in the right lane not to cut anyone off and turn into the right lane on the carriageway when I come off, instead of doing the standard thing of left lane to left lane, then indicate and move right.
I really think you should stick with an ADI but don't give up private practice, it's still good to have if anything else the parallel parking alone.