I just returned to work and can't stop worrying about my baby. How can I let go?

Call it love instead. Don't call it worrying about your baby - think of it instead of loving your baby, and then the answer is "don't stop". It is a great sign, although your employer may not think so, that you are so in love with that new one that it's difficult to stop thinking of them. Ensure you have someone you trust caring for them, and then relax, delve into your work, but sneak frequent peaks at pictures!
Keep in contact. Its very difficult to be separated from your loved one. If possible keep a picture of him/her on your desk, so you can see their cute face during the workday. Make sure you are comfortable with the childcare you have chosen; this will lessen your worries. Treasure your time at home with your baby and try not to take work home.