I guess my explanation was a bit complicated, so I am sorry for that. Since English isn't my first language, sometimes I have a hard time explaining what I am thinking. Let me try to explain this more clearly.

Before starting to draw your box, first, draw a Y shape, just like in the video uploaded by Uncomfortable. You can check the video from the link I gave you in the feedback. After that, draw some dots to plan out the trajectory of the new corner that you will draw next. You can see the dots in this diagram. Also using dots to plan out your line (for example, where does it start and where does it end) is explained in the Rough Perspective homework's video, so you might want to check it out. Those dots will help you plan your lines before drawing them and help you ghost the lines more clearly at the same time.

As for the vanishing points, you shouldn't draw them when you do the challenge, but instead, always think about their existence in the space. The box consists of several sets of parallel lines, and when we observe a box in perspective, the lines that are parallel to each other should converge to a specific vanishing point. So, for example, if we think about the front face of a box, there are four lines in total. If we divide those four lines into two sets, there will be two sets of parallel lines. So when we draw them in perspective, the parallel lines should converge to a specific vanishing point according to the rules of the perspective. Every set of parallel lines creates its own vanishing points. The three-point perspective, which we are studying right now, consists of three vanishing points. When you draw the box, you should find the lines that are parallel to each other and converge them to those vanishing points. It is really hard to imagine the place of the vanishing points since we are not allowed to draw them, but once you understand the theory behind it, you will start to draw more accurate boxes. If you have a hard time imagining the vanishing points, deepen the foreshortening and set your imaginary vanishing points close to your box. And try to ghost your lines towards your imaginary vanishing points, of course, without drawing them. This exercise is actually quite similar to the Rough Perspective Homework. The differences are that in the 250 Box Challenge you will not draw the vanishing points, and you will have three vanishing points instead of one.

To understand the perspective topic better, you can check Uncomfortable's explanation from here to understand what a vanishing point is and how does it work, and then, you might want to do the Rough Perspective Homework one more time. This might help you increase your understanding of perspective and vanishing points.

And for the health part, I hope you are doing better. I like the idea of having a schedule to organize your day. I'm trying to stick to a schedule a little similar to yours, but it is hard, and if you can do it then, that's really good. Studying how to draw, especially following the 50% rule really helps me stay healthy and happy. This way, I can enjoy drawing and build up enough energy to deal with other problems I encounter in daily life. Of course, sometimes we can get lost in our problems, and that's why I suggested professional help. I am saying this again, but it did help me in the past, and I would recommend it to everyone who has an opportunity to get one. And if it is not possible, finding people who are safe to share your problems with and talking with them is effective as well.