Most Helpful Girl
This girl is REALLY confused. It's best to just move on. She is hurting you and it is not fair. It's best to just hurt now and cut her off rather than hurt later when she continues with this behavior. At best- you guys end up back together, but you will always be insecure that she's going to leave you again, and that will only drive you mad. Just cut her off for now, don't even try and be friends because your conflicting emotions will make things hard. It's best to just cut her off for now, that way in a few weeks you can see her again and actually see her for who she really is rather than being blinded by your emotions.
Most Helpful Guy
Let's look at the facts.
She dated him a month and left him.
She dates you a month and leaves you. He's willing to get back with her.
You are willing to get back with her.
She leaves you guys wanting more.
She hasn't seen him in 2 years and leaves you for the possibility of him.
Seems like it's the excitement and novelty that gets her.
Think carefully why you like her (write it down even, what kinds of traits predominate)? You might have your answer.