Do you go out to work while your daughter is in nursery? maybe you have an issue with having to go out to work...maybe you don't enjoy your job, or maybe you do enjoy it and are feeling guilty because of that!
There really is no need to feel guilty about leaving your daughter in a safe place which she enjoys ~ however as a parent myself I can honestly say that being a mum is a life sentence of guilt..especially if you work.
If you're happy with your chosen nursery, and your daughter feels the same, then a short time apart could be of benefit. Your litle girl'll learn how to interact with others, and things won't seem so bad for her when she starts school full-time. I don't think you ever really stop feeling guilty about leaving your children, but if you have no choice, then at least she's in a safe environment, and she'll gain a sense of independence. Hope all goes well.