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.