Clement said:
What do you mean a branch swing?
-Clement
When a monkey is traveling through the canopy of a forest, swinging from branch to branch, it won't let go of the previous branch until the next branch is firmly within its grasp.
The analogy with relationships is that a woman, when she is preparing to start a new relationship with a new boyfriend, won't let go of the current/old boyfriend until she has already begun and consummated her relationship with the new boyfriend. This is typically the way women operate. They are petrified scared of leaving one perfectly good boyfriend and being left in the lurch in mid air if the next relationship with the new boyfriend is abortive.
I don't want to tell you to dump this girl out of the blue, becoz actually you might be playing right into her hands if you do this. Girls care VERY much about social protocol, and she knows that if she dumps you for a novel player just because, she will be frowned on. That may be all that's keeping you two together at this stage. If you dump her, not only does she get to breathe a big sigh of relief, she also gets to paint you as the bad guy, her as the victim, and the new boyfriend as the knight in shining armor who came to her rescue. That is not a productive situation for you to be in.
But, I do think you need to be preparing for the end of this relationship, as that is a distinct possibility at this stage. You need to be in a position, where, when/if she does dump you, you are totally fine with it. In fact, you're already set for it and are able to adapt to it better than she ever expected. If she dumps you, and then sees how quickly you moved on and how much better your life has improved so rapidly after that, she will regret her decision for the rest of her life.