Is your job, or your commute, new to you? If so things may get better with time, even though being tired and going slow is frustrating - I know I don't like it either but it happens to me sometimes too. I also have a pretty physical job, but it doesn't kick my butt like it used to when I first started. One more thought - I'd guess it's tough to get enough sleep during the work week. I start at 5am so I'm up before 4am every work day - I almost never get "enough" sleep, but feel better at work and on the bike when I get closer to enough.
Both are new to me actually. I've done landscaping and roofing for years, but this is my first "full time" job doing it. It was weird that when I got this job, the 6 days leading up to the starting date, all I could think about was riding my bike to work.
As for the sleeping part, yes, I never felt I got enough sleep. Even though my start time is 6:30, they want us to be there at 6:15 or so. So it leaves me waking up at 4:30 to get out of the door around 5:05-5:10 just to make sure I'm there on time. And then I'm usually getting home around 7:00 pm. I usually shower, eat, and I'm in bed by 9:00.
I'm assuming it will get easier, the work and commute, but the 2 days I did commute, and the suck I felt is not making me want to ride again.