You have been with your girlfriend for over a year, and then you asked her to move in with you. I wonder what happened to make you suddenly change your mind about this gal once she moved in? After over a year, you certainly knew her and her habits well. I don't envy you as you try to let her down easy. Understand what you are losing and decide how to explain your flip-flopping to her.
I think if you are unhappy in the situation, you should tell her so. If she is pregnant or has had a child, you should go through counseling together to figure out whether you can make a go of it as a family. But if she is not pregnant and you have no other ties, let her know now that you are not feeling the love. And stop sleeping with her or there is every chance she will stop her birth control and then you will be in a fix.
Don't tell her that you have regretted living with her for a year, it will make her furious that you have been lying to her. But do tell her that you can't find it in your heart to love her the way she loves you (and deserves to be loved). Don't let worries about "what she will do if I leave" run you, she is a grownup and makes her own choices.