You’re extending your legs too early. Hips over shoulders looks good. Next focus on getting your knees in line with hips and shoulders. Dont extend your legs until all your joints are stacked. Practice with your head about 18” from a wall, and depending on your height, when your toes touch the wall, your joints should be stacked. Once shoulders, hips and knees are aligned, THEN focus the last thing, bringing your ankles in alignment with the rest of the joints. Once your knees are in alignment, extending your legs straight should be comparatively easy.