2:53 PM, Thursday April 23rd 2020
If you aren't already doing warmups before you start, you'll need to start doing it. It helps you get into the right rythm and mindset to produce your best lines.
You should also take the time to sit down and experiment to find the the right combination of speed and pressure that works best for you.
I had similar issues with my lines having slight curves and missing the mark, and generally being inaccurate. I'd been ghosting and drawing my lines at really high speeds and at that point it was hard to get my hand to stop at the right time, and while the lines looked confident, i had very little control.
It was a surprise to me then, that when I tried drawing at slower speeds, my hand was significantly more steadier than when i'd started.
As you progress through draw a box, if you're doing it right, you'll build up mileage and muscle memory, which is why, again, I recommend you really sit down and experiment to find out how you draw lines best.