So -- you're feeling fed-up and discouraged and you don't think you can ever finish This Thing called a dissertation.
While every dissertationer feels this way at some point for some length of time, it's not something specific to dissertation-writing or Ph.D.-getting.
It's a part of the process of creative work (and yes, research is creative work!) and, maybe even more importantly, it's part of the process of developing expertise -- expertise as a researcher, as a writer, expertise in your particular field, etc.
And the development of expertise fundamentally means developing and elaborating related networks in your brain. And networks grow and strengthen by way of use, specifically frequency and intensity of attention.
Translated -- that means working on your research frequently ("touch it daily" is the mantra of my group members) and for periods of intense focus. That focus doesn't have to be for hours and hours at a time -- and likely shouldn't be, for a number of reasons -- but it should be real Focus and not multi-tasking every 30 seconds to check email, reply to a text, make a cup of tea...
But that's not very inspirational, is it? Let me turn to a creative writer - Ira Glass - to say it much better :-) ...