B.,
I have been through the same thing. I have a 5 year old and an almost 2 year old. I work full time. I cried the ENTIRE night the night before I sent my 5 year old to daycare (when he was 8 weeks old) and I cried most of the day after I dropped him off. I cried with the 2 year old too, but it was not nearly as hard as the first child. It is still difficult though, 5 years later (mostly on Mondays after a nice weekend with my kids).
You probably already have selected a daycare since you are about to return to work? Visit the daycare at odd times, not always when you drop your daughter off and pick her up. Sometimes visit over your lunch break, drop in or pick her up early once in awhile if you can. This will help you feel better because you will know how she is cared for all day, not just when they have an expected "audience".
Also, talk to the owner or manager of the center. They have a lot of experience with this and can be great sources of support.
In my experience, it takes a child about 2 months or so to get used to a center. After you are sure you have found the best place, and if you or your daughter just can't get used to it, you need to make a change. I remember the day I sent my 5 year old to daycare for the first time. Another mother was crying too. I asked her how long she had been sending her son there and she said 6 months. She just couldn't adjust. And guess what, she eventually quit working. But she gave herself a chance to adjust and I'll bet she'll never look back.
You will also be able to tell from your daughter if she being taken care of. The daycare where I send my children is wonderful!
There are benefits to working, as others have alluded to. I would hate for you to quit before you have had a chance to adjust to it. Take 6 months to work and see how you feel. Re-evaluate at that point. I am fortunate to work at a place which gives me some flexibility--I worked part-time for a few months--but I returned to full-time.
The women you work with at work will also be a HUGE source of support. Most of us have been through it and know exactly how you are feeling.
Also, talk to your friends who stay home. This is always helpful to me. I realize that they struggle too. Each woman has to make her own decision based on what is best for her family. For some families, it is to work (as hard as that is to do!!)
Give yourself a hug! This is the hardest part about being a mother. But you can do it! And you might even find it is a good thing for you and your daughter. It will make you stronger and you will definitely appreciate your time with her more!