One: Two weeks isn't long at all. It takes time for the muscle control to become instinctive.

Two: Your lines aren't bad at all. In fact they are quite good IMO. There seems to be a bit of bowing in the middle. They are quite long too so not a surprise if they are not quite there yet.

You need to get to the point where you are drawing confidently and not using speed to compensate for any inaccuracy. So note with your lines what your are unhappy with and just focus on the that for a few days. Re-evaluate and repeat.

People significantly underestimate just how long it takes to make quality lines of any sort.

Oh, and don't grind.

Those are some pretty damn good lines, my dude.

I'm reaching my first year of practice now and altough I can draw confident lines if I really focus, it's still something that requires a bit of concentration.

As Scoobyclub said, don't overdo it, build your muscle memory a little at a time. I personally do a page of Ellipses in Planes everyday as a warm-up to keep it in check.

The whole point of Draw a Box is to teach you the good mentality and posture to improve your art, thus why you shouldn't focus entirely on perfecting the exercises it provides you. It's perfectly normal to not nail all of them right off the go. Keep them in mind and practice them from time to time, but avoid grinding them.

Now, an advice I could give you is to remember to rotate the page whenever the need arises. In your video, you never seem to tilt the page at all. I personally draw most of my lines from an angle, I find it easier to bring in the full potential of your shoulder that way. Don't hesitate to try different postures and experiment.