I wanted to give a late welcome to the forums xD
but I also was really struggling with boxes, so much so that I looked into some "drawabox" exercises. I started getting it more after that, plus it helped to practice even if some of my boxes were major fails.
Something I'd recommend is to have patience with yourself when you don't get it automatically.
I was also struggling with my gestures; I kept drawing them too long or just not correctly.
For finding the line of movement, it helped me to pull up the reference picture, walk away from my monitor and squint. That made my vision blurry and I could see the reference as more of a silhouette, usually being able to see the line of motion in all limbs easier.
As for length and positioning, I pull up the ref again and study where things hit. for example, if the head lines up with the foot or a hand.
also keep in mind 90% of the time, the torso + head length is usually the length of the legs if the person is standing up.
I wish you luck and fun on this art journey :3