Fear of the unknown. I know how you feel. I get it a lot and I really resent having it, so I fight it. The jitters I had before all of my bike tours... oh my god! For bike-related adventures, though, I have one mantra that I recite when contemplating them: "It gets better on the bike". And it's true. Once you're actually out there and pedalling it usually doesn't feel so bad.

Anyway, as others said, your commute will soon stop being "the unknown" to you and you'll feel fine. Unless it becomes "an unpleasant known" (e.g. it is really bike-unfriendly). But from the way you describe it, it sounds not too bad. Just need to get used to it.