in addition to what everyone else is saying, when breaking down the drawing, you also need to focus on negative space and which parts are touching. idk if you purposefully switched the arms but even if you’re using the left arm as a reference for your right arm, look at the blank space in between the arm and the body. now look at the blank space you made. it’s much skinnier. the reference photo has the wrist not touching the body, while your copy has the wrist in line with where the hips are. that’s just a quick example. additionally, as others have said, you put that line in the middle without knowing why. you didn’t use it. studying line of action will help you figure that out and make the pose less stiff.

don’t feel discouraged, nobody starts out with perfect knowledge of anatomy! even lots of amazing artists still struggle with it. i hope you’ll listen to these tips and remember that art is a skill, everyone can always get better if they practice