Six straight blocks of heeling is a lot. For just four months' work with no prior training and a four-year-old dog who's doubtlessly had plenty of time to develop other habits, it's probably too much to ask. I don't know any dog who would have been able to do that.
I do think your expectations are probably over-ambitious for the dog you have and the time you've had her. Relax. Scale back a little. You'll get there eventually, and in the meantime, focus on the things you enjoy about your dog. What is fun with her? What are the moments you love? If you lost her tomorrow, what would you regret not having done today?
ARCHX TDCH Pongu the Insane, CD-C, RL1X3, RL2X2, RL3X (GSD mix, b. Apr 2010)
Crookytail the Tigerwuff, RL1, NTD (Akita mix, b. Jan 2011)