Having read your thread I think you already know what you really want to do.

You don't sound happy with the place where your relationship is and you don't seem to think that it will get any better. Only you can make the decision about your relationship but you need to take a step back and decide what you want from life, are you happy settling for something where you don't feel like you're a team and appear to have already talked till you're blue in the face without actually resolving anything? BUT, and this is the hardest thing, you have to take this decision thinking about you and only you, you can't decide not to split with her because it will make her unhappy (to be harsh she'll get over it, everyone does eventually) and not because you want to try something new with this other girl (I admire your actions though I wish my ex had been more honourable).

With my ex and I it ended up that we were always talking and trying to sort things out and I think that he was sort of where you were. He just didn't want to be there anymore. To be honest I wish he'd been as straight as you appear to have been with your girlfriend. In the end he ended up cheating on me and told me a whole pack of lies so that I misconstrued any comments I overheard and then on deciding he didn't want to be in a relationship with me anymore decided that, as he was posted to Germany, he'd let us grow apart but that's a whole other story.

SOrry this is sort of a long post for a very simple answer which is really just that you need to sit down and make a decision, it sounds like you aren't happy in the relationship and don't think you are going to the same place - a good enough reason to call it quits for me. You then no matter how hard it is need to stick by the decision, NO MATTER WHAT.

If you do decide to split up, you then need to take some time out, be single. DOn't hook up with this other girl on the rebound just cool it, have some fun, take stock and then decide on that one.

Hope this helps.