C.D.
C.,
I know this is frustrating for you; both of mine went through this, too. Here's what I did, and it really helped:
Look for something of yours that your daughter sees you use or wear every day - an inexpensive watch, a scarf, something that is easily portable. Even if you have to go get something inexpensive and "use" it every day. You can talk about it with your daughter, and tell her how important it is to you. Keep it with you when you go through your day, have her "take care of it" for you when you go to the bathroom, do the dishes, take a bath, et cetera. Have it with you when you go to the "Y" or to church.
When you take her to the nursery, ask her to keep it for you, and reassure her that you'll be back for her and your "watch" soon ... then leave the item with her when you go to your class or to church service. She'll know you're coming back for the item, because of the pattern you've established with it and her.
At this age, children develop the awareness of their being separate from us, and do not understand their own relevance. But they know we will come back for our "very important watch" ... or whatever the item is. This stage won't last long, tho' it seems like forever when you're going through it!
Hope this helps.
C. D