I for the life of me cannot get my head wrapped around this problem

If there are no lines on the ground I do the same thing, touch driver's side to the dock and varying degrees of space on the passenger's side.

Fast forward to the end--

 

I really really appreciate all of your help, everyone. I know I look ridiculous pulling up 15 times to get straight. But oh well. If this job was easy, everyone would do it, right?

As for those docks without lines... I bought this 100' piece of reflective rope. I'll typically just lay it out from the bump stop on the dock. Gives you a nice bright line anywhere you go.

https://www.amazon.com/gp/product/B010EJ52BI/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

 

It's tricker than it looks to back up to a wall with no frame of reference on the ground like a painted line or seam in the concrete to tell you what 90* looks like. And sometimes they don't paint the lines perfectly 90* either.

 

So what if your off a couple of feet? The product hoses are flexible.

Opps sorry, my bad
. Wrong trailer .

 

Get it just right and you don't have to move when changing between main trailer and pup.

 

It could just be a slight distortion in the mirror, I have 15 Cascadia, and the right side always looks off if I try to line up on it. I just compensate for it now and line up on the left mirror without thinking about it.

 

I had the same problem for a while. Now I focus more on lining up with my P/S mirror and I feel crooked but im actually straight. If I line up with the drivers, I feel straight but I'm crooked just like the OP's drawing. I figured I was just stupid. However I haven't ruled out that possibility either. OP, are you in a cascadia by any chance?

 

Apologies for the bump. But yes it is a Cascadia.

 

This is super common, especially since we tend to look primarily at the left mirror while docking, and is more extreme when there are no lines painted on the ground. Then you can get a follow the leader effect at those places, where every truck is angled to the left.

You can get square even at those places by looking the angle between the side of your trailer and the wall in both mirrors. They should both appear to be greater than 90 degrees to the wall, like this:
___
\ /

The problem is when you focus on the left side, shooting for a 90 degree angle. It is a 90 degree angle you're shooting for in reality, but it shouldn't look like one in your mirror. If it looks like you're square in your left mirror, you will always be angled to the left.