Try to keep in mind the weight of the object you’re drawing and the gravity. If the line dips down, the object likely has gravity pulling on it there and you can make the line thicker to emphasise that. Same with shadows, if it’s a darker area you can make the lines thicker and thinner based on where the light hits. If lines join together, you can define them by thickening them at/near the joint. The lines I’ve used here are quite exaggerated and comic-bookish but the advice still stands for softer drawings like yours. If you want to practice I would try putting in as little detail as possible on things like the hair and using the line weights to show what material/texture things are. Try to focus a little more on each line instead of sketching them quickly, and use a brush with more variation for practice. Hopefully this was helpful :)