How can I stay motivated and become more productive as a video editor?
Do you drink coffee?  That's my solution.

More seriously though - For me, it helps to jump from project to project, setting individual goals each time I move to a different project, e.g. for project 'x', finish creating sub-clips from raw footage before I stop editing.  It also helps to alternate between tasks that are boring and tasks that are more exciting.

In reality, you often don't have the luxury of moving away from a project or boring tasks.  That's when you have to just power through it.  In the end, if you're proud of what you have done, it's always worth it. 

It can be extremely frustrating to work with bad material or to have to work in projects with missing shots.  Again, with these situations, there is no secret sauce.  You edit what you can, and do your best to prevent the situation next time. 

I hope this helps. 

Calvin