The partying and stuff is her way of dealing. No matter how much fun she looks like she is having, she is suffering as well. We all want to look like we are fine without our ex and that we can live without them even if we are dying on the inside. Not to say that she "all the sudden" changed, I'm sure she is more or less the same person she was when you guys broke up. We have no control over how we feel, but we have control over what we do and I think when we try to do new, spontaneous, sporadic things, it makes us feel like we have control over our feelings (I'm fine without them, look at me I'm having fun).
But at the end of the day, when they are back home alone, guess who they are thinking of? You. And that's when you get the texts like I miss you and etc. etc. As hard as it is to not read into them, you have to look at their actions. Their actions are still dumping you, still being without you, and still doing their own thing. If she wanted to be with you, she would be. Not that now is a good time to be back together anyway because of how emotionally shitty you both are feeling because things will be back to normal in a blink of an eye and she'll want out again. But you get my drift, right?
They are right: she says these things because she wants to feel like she hasn't lost you. She wants to have you around as a cushion so to speak so she doesn't have to do this alone. She wants options, and she is only concerned about herself and her feelings. That's how it is in the wake of the break up, you are looking out for only you. So what's in your best interest? Getting her out of your system and rediscovering how to live life without her, even if you don't want to. Remember who you were when you guys first fell in love? Do you see yourself as that same person now? No. And that person that you were was who attracted her in the first place. That should be your goal.
And it's not going to be easy. You will hit highs and lows, ups and downs. You have to engraine it into your head that while she is the only one that can make you feel better, she's not going to, nor should you have to depend on her to do so. You have to push through this yourself and do whatever it takes that doesn't involve her to make yourself feel better. She is not the only person on this planet you can be happy with and you will realize this in due time.
Waking up next to a beautiful girl,
Step outside and say hello to my beautiful world.