When I started drawing, I had the same problem with short sketchy sections. The one piece of information that clicked and made me ‘get it’ was to draw from the elbow. My instructor made me keep a stiff wrist and have all movements come from my elbow. That forced me into longer, more confident strokes. Start with simple shapes (cell phone, remote control, etc) that will allow you to gain confidence and line consistency. From there you can move onto come complex shapes and details.