Most Helpful Girl
"It's not you, it's me" pretty much means that her feelings for you are gone and she's moving on. It's never smart to stay in touch after a breakup because you give them the satisfaction of not having to feel bad for you. If they don't feel bad, they will not think of you. If they don't think of you, they will not miss you. If they don't miss you, they will not start comparing what she had to what she has now. When she finds that she had it good with you, they return to you. I would say cut off communication now. Do not even tell her. Drop off the face of the planet and either date or do something productive. Let her live out this relationship and give her the opportunity to miss you.
Most Helpful Guy
I know it's really frustrating to observe this. You thought she was going to "stay single" like she said she would and it almost felt like you had this little project together. And then she has the nerve to get involved so quickly...
You can learn a great deal in this situation if you'll focus on yourself and not her. She is completely immaterial. She broke up and it's you job to find peace within yourself. How peaceful can you be? How much can you let her go?
How much can you free yourself from the need to "know": is this serious? Is it rebound? Will it last? Then what's going to happen?
There is no way you can ever know what's in another person't heart, not ever. There is no "rebound". That's just a word invented by frustrated lovers who needed to put their exes down. "Oh, she's in rebound mode. It'll never last". But deep down, they just feel badly about themselves, and they feel hurt.
So, your job is to focus on YOU. Learn to relax your mind, learn to meditate which is really easy. You will realize that you have everything you need. Also, if you can, begin to just think back to the times you were happy together. What was it that drew you to her in the first place? What do you really like about her? Etc.
These thoughts, when stable inside you, will make it much easier to let her go. And you might as well, because she is gone. But what you can learn will lead the next one, a MUCH better one, to find you. That's just the way it works.
Right now, you don't really want her back. How I know this is because if you could teleport her over there with you right now, what would happen? She's in love with another guy, so she would leave you instantly again.
And you don't want a girl who leaves you, you want someone who wants you and who will stay. Likewise, she would never want someone who got so devastated by her leaving. She would want someone who wants her but who was basically happy in his own skin.
Does this make any sense? I hope it provides some perspective.
Best of luck.