I won't say that proko is bad, but his videos sometimes lack some depth.
If you want to get acquainted with perspective, there's only one way:
Draw a lot of boxes on a page, with one vanishing point, and from that point make some floating boxes going up, going down, tipped to one side...etc...This will help you.
when you are familiar and comfortable with this, you can switch to 2 points perspective, and repeat the process.
Draw streets, boxes, bottles, in any angle, making sure you detect where the horizon, so your piece's eye level, is, and you'll get an even better vision of things.
I hope you'll get through this pickle. :)