It can take couple of years or even more to improve line quality, so don't worry too much about it. If you do line exercises for 10-30 minutes everyday you should see improvement in a year. One thing I noticed in your drawing is that you are putting too much pressure and all your lines are way too dark. Try loosening your grip and try to sketch lighter, only making your final lines dark. Specifically try not to hold your pencil too tight as that will mess up your wrist.

Cute! I also struggle with scratchy/sketchy drawings... And I would tell you how long it took me to overcome that, but I haven't really! I just got better at hiding it haha. Learning to put down a single, carefully planned and measured mark just once and then live with it is extremely difficult and rare to accomplish! Almost nobody is capable of that level of draughtsmanship, especially for those of us who like to scratch. So my advice is to never be afraid to erase a bad batch of fuzzy lines and redraw with better control. Sharpen that pencil! Erase ruthlessly and without hesitation! Draw from the shoulder and try experimenting with an overhand grip. For me, a typical clean finished drawing is built upon layers of repeated redrawing. As long as you're not pushing too hard and grinding up your paper, your results will be better for it. Hope this helps!