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

If there is a line to follow on your left side, are you trying to make it look parallel with your trailer and drive tires in your mirror?

 

Are you right handed?

 

I actually used to have the same problem. I eventually started making it a point to line up to where I thought I was straight, then overdo it a little so it would appear, in my mirrors, that the trailer is ####ed slightly to the left. Then I get out and lo and behold, it's straight as an arrow.

 

Are you moving your body around while looking in mirrors? If so try keeping yourself sitting still swiveling head rather than body.

 

Usually doesn't matter, it can still be unloaded or loaded. but if you must, make the affort to change the angle

 

You have to consider perspective.
What is far behind you, as in the rear of the trailer. should look closer to the line than your front end.

When looking back in the mirror the line should not be parallel, but at an angle.
Looking back in the mirrors the lines should look something like this...
\ /
It is the reverse of what you see of the road in front of you when you are driving, which is...
/ \

If you use both mirrors you will see the difference in angle very clearly.

 

I've had the same issue. Eventually I just started compensating for it. No problems anymore.