I figured I should give an update, and thank you all for your wonderful advice.

I found a Novara waterproof windbreaking jacket that, while rather large on me, breaks the wind very nicely and makes a huge difference for my ride. The weather hasn't been as cold as it was before, so I don't know if it will stand up to near-freezing temperatures, but so far it has been great.

I've also gotten a better water bottle and am drinking more on my ride: I've noticed a big difference there, too. Thanks for pointing out how important drinking is during the ride.

A third difference is that I've changed my commute so that I don't ride through downtown anymore, and I have fewer lights to stop at. Most of the ride is along the river, and while it gets windy, I stay warm because I don't have to stop.

Overall, I think the biggest improvement is the windbreaking jacket.

Thanks again!
Sara