Prefacing this first!
I've not taken this course and I'm not an expert!
But to the points already made above, I don't think your squares look off. And I have an additional note.
What it could be is what you're seeing is that as the Left, 1, rectangles/square "go towards the background" the perspective is changing. If that makes sense.
If I recall, as objects get closer to the horizon line, or "farther away" they're smaller. And with your case, those vertical left lines are "shrinking" (see my screenshot)
And they probably look stretched because of how much you're "distancing" the bottom horizontal lines (one is circled for example in screenshot.)
If you wanted to fix it, you could make your bottom lines a "shorter distance"

Like I imagine your "platform" or set of boxes is like an accordian that needs to be "pushed" from its left side toward the right a little bit, if we're thinking visually.
Hope that helps!