Two slightly different lines to a Chevron

Today I got to watch another rider's lesson and then ride some of the same fences.  I arrogantly thought that if I watched someone else be taught first, John wouldn't have to tell me any of the same things, but I was wildly wrong about that.  However, it was interesting to watch, to see what I could see in the horse (and rider), what John told her to do, and how much of a difference it made.  It was also, similar to a group lesson, interesting to watch what challenged her and how John worked with her to adjust it.
We did a bit of warm up while she was taking a break, and John's first comment was to make Duke more supple; that I was (my words) basically just puttering around the arena.
He put us right to work when the lesson started, jumping a vertical on the right lead.  I had watched the prior rider and was watching how she made the circle - i.e. where she turned to try to get to the center of the jump.  She frequently cut the circle too short, and would jump the fence at an angle.  I, on the other hand, cut the circle too wide (Duke drifts to the left) and so would end up next to the far standard.  Even though we worked on this last jump lesson, and I put out ground poles and rode to the center of the ground pole from various starting points, I still struggled with getting the turn correct.
John told me to use my right hand to bend his neck, and then my left leg to keep him from drifting out.  Afterwards, he said that this is a good exercise for show jumping in the warm up on Sundays, because just going around and around in the circle helps loosen Duke up and keep him easy, so he doesn't grab the bit and run on the far side of the fence.
From there, we rode a three stride line, which was a vertical to vertical.  Although we were still a bit left, this was easier to line up because I had more space to get to the first fence compared to the circle.
Then John had us do a bounce, that was a cross rail - cross rail.  Duke didn't even bat an eye at it.  John made it a cross rail - vertical and Duke still didn't bat an eye, so then we went down the center line and jumped the Chevron.  No eye batting.
But the next series of fences were a challenge.  John had a narrow green/white over a barrel, four stride, Chevron on the right lead, and then a narrow plank over a barrel, bending line to the Chevron. The plank to Chevron had to be lined up just so from when you started to turn towards the narrow, but the vertical to Chevron was an easy straight line, except it was on the right lead.
So we blew the vertical pretty much every time, but did ok with the plank pretty much every time (instead of making a bending line, we just jumped the narrow plank at an angle, so we had a straight shot).
BUT, I could not turn left after the Chevron.  We'd either skid around on a 10 meter circle, or head straight towards the wall and then suddenly bounce left.  It was frustrating.  John suggested thinking that we were riding on a circle, and I just ripped poor Duke around on two legs.  So then he suggested thinking trot first, and we almost hit the wall.  He had to bring us back to circling over the far vertical (going to the left) because I was just getting frazzled.
Duke, however, bless his little heart, just kept trying, even though we ended up knocking down the standard (with my foot), the Chevron, the vertical, the Chevron again, etc.
What the right turn was ended up being so simple - I wasn't making the correct turn to it, we were drifting left, a narrow is too narrow to drift left, and so we didn't have the line correct to the Chevron. Every damn time until John told me, then we jumped it fine at least twice (maybe three times) in a row.
Why couldn't I figure that out myself?  He told me in the last lesson.  He told me in at least 10 lessons before that.  He told me WHILE WE WARMED UP OVER THE VERTICAL, and yet I immediately reverted to the exact same mistake and never once figured out how to correct it on my own.  I'm a dimwit.
Duke is a sweetheart though.  I was super happy with him.  He just kept trying.
John said his weight is good, that he'll lose his fat as the competition season starts, and that since it took almost 10 months to get the weight on that he lost when he traveled cross country (in the photos of him at Jane's, he looks a lot more like he does now), we don't want him to start losing weight because we won't be able to get it back on.  He said he looks good - more filled out.
It was a good lesson, not because I showed I'd done a great job working on my homework since the last lesson, but because how and where I'm stuck hopefully finally got into my head, so maybe I'll be able to problem solve this for myself (or at least recognize it) going forward.