Hey so this is something I've definitely struggled with over the years, it can feel like you're just treading water, spinning your wheels or running in place. What I came to realize is that the more I would focus on "perfect" application of everything I know or in your case learned during the process itself, the less I would produce. The truth is we're always going to be improving, with our eyes (usually) developing way faster than our skills. It can make those longer projects feel like you ultimately got nowhere by the end. The real irony is if you never even finish an 8+ hour project then you truly did get nowhere.
My suggestion is to just focus on completion. Period. Constantly restarting or leaving several projects unfinished is a terrible habit in itself. Push through the pain of the "ugly" phase of a piece and then analyze what worked and what didn't once its complete. You'll get so much more mileage and workflow optimization this way.
As for your thumbnail problem, perhaps you should spend more time in the thumbing phase. Once you have say, 5 that look good, put a couple hours into each. Then choose your favorite and render that SoB until it's complete. At the end of it all reward yourself by compiling everything on a nice big canvas and analyze all your work, every step of the way. A side note, it's also a good idea to do some shorter projects as well. Maybe your scope is a little too big and you get lost a long the way, especially if you haven't actually completed many projects as is.
Anyways I've rambled on enough. Hope this helps