We all go through this, at some point or another, and in one way or another.

I would not say to slow down as a rule, but there is absolutely nothing wrong with a leisurely ride.

Last night I did my commute (about 17 miles) at 19.2 mph. I avergae around 16-17 on normal days, and there are days I do 14-16. It all depends on how my legs feel, how tired I am, how late I am, what I ate for breakfast, the weather, traffic lights, etc...

What I'm saying is, there is nothing wrong with a nice cruise, but also nothing wrong with going as fast as you can at times. I find mixing the two of them up to me a more effective method anyways.

One other thing you might try is just riding different gears on different days. Some days I push hard gears, others, I spin smaller gears. Both make you a better rider but use different muscles and other associated body parts so it's nice to switch off.

Lastly, I'm not sure how long you have been riding, but the body adjusts. If you consistently commute, you should lose that 3pm zombie stare at your desk after a few weeks. Your appetite should also stabilize.