To quickly answer your first main question, I feel that it gets easier over time to focus less on results and more on learning the many small and important facets of drawing/developing intuition. I made it a goal to actually start drawing in octoberish of 2019, as a College grad with 0 experience in drawing apart from a stick figure here and there. Even today, I will still look at the types of works I would like to achieve, and still fall into the trap of getting frustrated when I underperform (by a massive margin might I add) to what I want. The only real difference between then and now is my ability to more easily brush it off and keep trying. As Uncomfortable mentions, drawing is a step by step process, you want to handle problems one at a time, and being able to improve your abilities to do so in such a manner is invaluable. Once you have gotten far enough into the course, you will get to a point where you can look at what you are capable of now compared to before, and that was a MASSIVE motivation boost for me, even if my drawabox exercises looked like crap.

As a reply to this question, it depends. If you can see you are making a certain error over and over again, and you know WHY it is wrong, such as constantly getting the one vertical line on the opposing side of a box incorrect, you can start working on that and you will improve with that over time. If you know that you are doing something incorrect but can't quite figure it out, even after thoroughly thinking about it yourself and trying to find it, critiques are invaluable. I am currently experiencing this myself, with colors and lighting. I KNOW something is wrong with how i apply color and shading and I can't pinpoint it so I am currently trying to gather some critiques and information from wherever I can.

And be sure to do your best to follow the 50/50 rule, as it can allow you to focus your result oriented attitude on something else rather than your exercises. It cannot be emphasized enough that the drawabox exercises are just that, exercises that allow your mistakes and problems to be laid bare so you can see them and consciously work on improvement. If you are stuck in a result-oriented mentality, the 50/50 rule really can feel like anathema, you start drawing something and in your mind you already know what you want, you start drawing and the more and more it does not look like the final product you envisage ,the more frustrated you can get and you just stop. I fell victim to this up until I reached the final stretch of lesson 5 about 2 months ago. With what I have learned about perspective through drawabox, i have started drawing things like cityscapes or buildings, stuff that I can make, and look something similar to what I would like to achieve for the 50/50 rule. My main goal to draw things like characters, and be able to illustrate scenes in color, but by putting my focus in one aspect of drawing at a time, I can see and feel my skills improving. I am nearing the end of the drawabox course soon, so I am branching out and trying to learn things like color and shading, but I am still putting the vast majority of my focus into learning perspective and being able to manipulate shapes and forms in 3d on paper.

Sorry for the wall of text, about 11 months ago I was in your shoes, and I only changed about 2-3 months ago so I know how you feel and it SUCKS.