i have the same problem with riding to work. Its only 6 miles one way and takes me about 25 min. What I have come up with is that I am getting burned out and bored with the ride. Its just not fun. The way I go isnt really urban jungle like either so there really isn't a challenge.
What I did to partly solve this is to just stop riding to work. I took off about a month in July and part of august. It seems to work for me anyway.
I dont really know what to tell ya. sorry