When you're drawing, try to break the thing your drawing down into simple shapes (circles, ovals, rectangles, wedges, etc.) and lightly sketch those first before you start working on details and subtleties. That will help you work out the proportions, overall shapes, and composition before getting too far into the drawing to fix any of that.
Also, it'll help teach you to see shapes and their relationships. My belief is that drawing is all about seeing. What goes on with pencils and pens and paper, that's secondary.
If you're working with pencil on paper, you do this light sketching by just drawing very lightly, barely touching the paper with the pencil. In digital media, you can do your sketching on a separate layer.
As to feeling like your drawings aren't looking like what you're seeing in your head, and getting frustrated with that, here is a good short video on being creative that'll be relevant to how you're feeling:
https://www.youtube.com/watch?v=3ResTHKVxf4