Okay, I'll take an alternative viewpoint here. If you feel like driving to work once in a while, then go ahead and do it. Listen to the radio, drink the coffee, and relax. Nobody is keeping track of how often you ride but you. There is a long winter ahead of us, and if you hold yourself to an unreasonable expectation, you'll burn out. Sounds like you're just about burnt out already.

I get tired of riding sometimes, or don't feel like riding in the rain, so I take the subway. Usually by halfway through the morning commute on the subway I'm dying to get back on the bike again.

Variety is the spice of life - heck, maybe leave 4 hours early and walk sometime.