i'm in a very similar situation...
i'm finishing the 6th year of my phd (not counting an unofficial year maternity leave).
i really don't like what i'm doing anymore - i did like it when i started. the first three years were smooth. over the past year i've had recurring doubts and am nearly ready to quit. i've managed to surmount them each time. i have talked to my supervisor on numerous occasions and he simply says it's a decision only i can make.
in june, my supervisor said that i couldn't afford to waste any more time and if i had another lull in my writing schedule, then i wouldn't be able to finish on time. now it's been three weeks with nothing done. i'm miserable. i can't take advantage of my "real" life because this hangs over me constantly.
i've over half of the dissertation written. i find rewriting a terribly excruciating. my phd is a very big project. i have much too much data and archival material (it's a historical case study). i think i'm in over my head. how do i know if i can finish? how do i know when i should just quit and move on to something else?