Well first off, you have to think about your old relationship with your ex. Why did you break up? Who ended it? Was it because of some trivial thing, or was it because you two were just too different? If you honestly, truly think that you can talk and work it out and communicate better, then you need to call her and tell her that you've been thinking about her a lot, and that you regret your breakup. Then let her think it over and give her some space. Now think about your new GF. Find out how exactly she feels about you. If you say that "it's just not working out" for you, don't mention your ex 'because no one really wants to just be a rebound, and because you said you still care for her and wouldn't want to hurt her feelings.
