Please don't go back to your ex-wife simply for the sake of your child. You'll be a much better father if you're happy and fulfilled in life. Only you know how you really feel about each woman. But keep in mind that your ex, having newly exited the cult, hasn't taken the time to get to know herself or what she really wants. It's not good to make decisions out of fear, but there's a good chance your ex will wind up leaving you if you two get back together.
If you really love your girlfriend, then stay with her, but be involved in your daughter's life. Let the ex down easy by telling her that she has a whole new life to explore and that you're leaving room for her to find someone else to be in the place that she thinks you should now be.