If looking at other people's submissions gets you discouraged then don't do it. Comparison is the thief of joy.

2 weeks is not long enough to see much improvement. The 250 box challenge alone is not necessarily enough to see much improvement. Each person is at a different point in their journey going into the challenge. Sometimes people are close enough that the challenge allows them to break through on understanding some concept or some skill plateau. Often it doesn't. That doesn't mean you aren't capable of improving or not cut out at drawing. You are just starting at a different spot than they were.

Often it takes exposure to many different approaches to get someone to improve. For me, getting a larger pad of newprint and doing large loops, swirls, circles, etc with a thick oily crayon at the end of an extended arm's reach helped me really drive home the physical feeling of drawing with my whole arm.

In my expereince it also a matter of the physical development of your shoulder, arm, core muscles, and your mind/body connection. That is to say, this is a physical activity and so takes a certain level of physical conditioning and bodily awareness before you can break through; not unlike a sport. I believe that is often the answer to the mystery behind why a lot of people can sometimes take noticeably longer time to improve than others.

The 250 box challenge is not the final boss of mastering perspective, it is just a mini-boss. It is about mileage, not footage. Tempering your expectations can keep you from sabotaging your motivation.

If you have done multiple hundreds of boxes then you have to be nearing completion. Try to take heart in the fact that you have almost completed the challenge. You have already nearly finished one of the first big obstacles in this course that cause a lot of people to fall off.

Anybody doing the 250 box challenge should not be using a straightedge on anything but the projection lines that are simply there to check their work. So no, don't use a straightedge other than that. Freehanding is required by the assignment.

If you are wanting more specific advice than that then I suggest sharing some examples of your work.