Dear revolution, unfortunately, I have to agree with my fellow-forum members here.
It looks like she has the best of two worlds and is using it to the fullest.
You can continue this, as she's so sweet - as you say, but what will this bring you?? She has given up nothing, and you have given up your peace of mind, you changed your shift, and probably have given up more time and energy on this than she has.
Please look at this from a different perspective and don't feel a single bit of guilt if you see it the way we do. It takes two to make things work, and she's not going to give up anything - so please take the initiative yourself and go on with your life. Change your shift again, and get to know some other people OUTSIDE of the workplace.
Relationships at the workplace generally don't do so well, and can damage the career and reputation of one very quickly.
If you read some of the older threads regarding relatioships started at the workplace, you'll agree with us that this was a mistake, and that you still have time to take life in your hands and start living it.
Please keep us posted and let us know what your choice in this issue is, we are here 24/7 to try and help you through this.

Those of us who have 'been there, done that' wish you all the best.