The Robot Hunter

I know it's easy to say (and cliche) but the grass is always greener on the other side. If you and the ex-girlfriend are two different people now than you were when you dated before - if there's been maturity and some drastic way that you both feel about each other then I would say go for it! Win her back and don't give up! But if there hasn't been that maturity on both sides then chances are if you two did get back together it would be the same result. It would feel great to begin with but probably would end about the same way and you two would just hurt each other again. If the new girl isn't distracting you in any way from the last girl then I would say it's safe to say that you will probably end up hurting her too if you can't give up the idea that you and the ex are going to get back together.
Whatever you decide just remember what it seems like you learned from the first experience, that it's not just about you and the actions you take effect the feelings and actions of other people.