I'm sorry you are feeling this way. I want to add that I don't think you are off your rocker for feeling depression. You probably aren't getting enough sleep at night. You've still got hormones surging. And it is understandable that you would be very sad about having to go back to work. That is a natural reaction. Add all of these things together, and it is very easy to want to cry all day at work.
You will get through it. Your baby will be fine in daycare. Your baby will still have a bond with you. S/he knows who momma is!
So, don't feel like something is wrong with you. You are going through a normal, difficult time. Maybe your doctor can give you a little medicated boost, if you need it. Or, maybe just talking with people and recognizing that maybe you are just going through a period of grieving will help.
I don't know what your financial situation is, but I know I've had friends get very frugal and creative, (albeit giving up some luxuries) and be able to cut hours at work. There are library books on the subject of living simply. (I find reading time when I nurse).