Observation and construction are two different skills, thus they require different approaches, you cannot apply observation techniques to a tridimensional construction without considering how that construction will then have to be changed, otherwise you risk flattening that construction. So keep in mind that Drawabox focuses entirely on developing your spatial reasoning skills, put aside what you may have learned about observation and only make use of the concepts and techniques introduced in Drawabox.

It's not important that your constructions resemble your reference, your reference is just a base for you to apply the construction methods and develop your skills, the most important thing is that your constructions feel tridimensional.

It's completely normal to feel like you're struggling, as you're going from an entirely 2d technique of making art, to something that forces you to consider how the structures you draw exist in 3d space. However you should not attempt to tweak any mistakes, that's a problem in of itself, if you were supposed to fix mistakes until you got something "good" or "perfect", then you would be able to use an erasable medium.

But Drawabox wants to instill in you a respect for every mark you make, the idea that you must put your all into planning your mark before you execute it, thus once you make your mark it's done, if you go in with the idea that you can tweak the mistakes you'll make, your initial lines will become more sloppy and less carefully planned, so never redo, or attempt to fix anything.