Biking in the winter more difficult?
I bike all winter long, down to -30°C, there are some definite tolls to your bike at colder temperatures. Unlike a vehicle that warms up after the engine has been running for a while, bikes stay cold when you ride them cold. There are two things I notice the most, air pressure is definitely one of them; cold air shrinks, and low tires drag, so if you're riding dry roads I would certainly check to your pressure. Normally I don't put extra air in my tires, because up here the cold is usually accompanied by snow, and low tires give you better traction. The other thing I notice when it gets really cold is the grease in my bearings gets thicker, and as a result my hubs don't roll as fast. Pick your bike up and give the wheels a spin, you might find that they slow to a stop a lot quicker. I notice it in my headset too, but that doesn't affect speed.

One thing I've done to avoid drag due to thick bearing grease is to actually replace the grease in my wheels with Phil Wood Tenacious Oil. I tried it out as an experiment one year, and I've never had problems with it since (I kept checking inside the hubs for a while expecting to find them dry and rusty, but Phil's oil lives up to it's name; it's tenacious).  

Of course the best thing to do is to not push yourself, compensate for the conditions, leave a bit earlier, find a new gear range that you can maintain a comfortable cadence in without giving yourself a heart attack, and just plough along. If the conditions are miserable already, there's no sense in making yourself more miserable.