Rom, based on this new info you gave I'll tell you exactly why I think you two shouldn't be together.
From what you wrote, it's obvious that you are two very different personalities. She-- overachiever, has her life well on track as far as society is concerned. You--in a bit of a rut at the time, not someone who's "going places".
She tried to stimulate you to improve yourself. But she did it in such a manner that got you on the defensive. This to me sounds like she is a person with poor people skills and little or no patience with people that are not like her.
And tell you what? When a girl dumps you because she can't handle you at a certain point in life and then comes back when you've changed a bit more according to what she thinks things should be like, well, then I get all irky and irritated.
Sure, if you were some lazy drug-addict, abuser, molester or what-not in terms of unnacceptable behaviour, or someone with psychological disfunctionalities or issues, someone in need of therapy but insisting there's nothing wrong with you, making you impossible to live with, then sure, by all means, a person can and should set terms for the relationship and your behaviour by leaving you and coming back when you clean your act up.
But that sort of thing wasn't the case, right? It wasn't as if you were a complete wreck, were you? I think you just didn't really know what to do with your life at the time she decided to dump you. As you said earlier: you weren't driven in life. That's not a crime. And she dumped you for that. She dumped you because you weren't the little gem in her ring she wanted to parade with.
Did she know how to motivate/support you properly to improve yourself? No. She got you on the defensive. To me that speaks volumes about someone. Mostly that they have little tolerance or understanding for people not like them. Such people act in ways that get people on the defensive.
Of course I can easily be off the mark, basing myself on your limited info, but still. It sounds all too familair. This girl simply seems like she's not a person who's there for you. It rather sounds like she's there for her and bails the minute people need her in some way or another that she doesn't know how to deal with. (And you'll probably say that this is not true because she put up with you for a long time, but that's not what it likely was -- more like she didn't have any better prospects at the time, so she swallowed her irritation as best she could. And still she got you on the defensive.)
Is that what you expect or want from someone you believed you also had a best friends relationship with, aside from a romantic one?
Imagine yourself in a few years when you hit a depression perhaps, or some other dip in your life. Imagine you're seriously together again. Will she stand by you? Try to help you through it? Lovingly support you and stimulate you to seek help? Or will she do her little act again that gets you on the defensive once more -- and ultimately drive you two apart again?
Because that's what people do who get others on the defensive: drive others away with intolerance. Ask yourself why someone gets someone on the defensive. It's because they're usually not the most empathic and supportive people, who can't tolerate it when people don't behave as desired, or when things don't change as fast as they want it.
So with this in mind, think hard about wether this one is one for you and if you should pursue this further.
I know it's difficult to simply say goodbye to the good things that were. I've once known a girl who seemed a bit like yours and she was really a cold, calculative piece of work. We got along great, best friends you could say. But as sunny as things seemed when it was all well, it turned out there was no place in her heart for any difficulty. It had to be all be flowers and sunshine, or else bye-bye. And when you're suddenly faced with such a contrast, that's a big nut to swallow.
For a long time I tried to fix things, but ultimately you realize you're fighting a battle you'd better withdraw from to regroup. It's hard to realize this when you're close to the fire with so much fire for the other person inside you. Especially when you got along so well otherwise, and everything was great -- except for these one or two little things that seemed insignificant and seemed like they were things that could be overcome. But usually those one or two seemingly little issues that seem like hurdles you can take, turn out to be mountains one or both simply don't want to climb.
I'd like to think that people should stick around for eachother, also when it's difficult and demanding of you. Not bail when it gets a little tough. If people do, that's usually a sign you're not compatible enough.
Anyway, I hope this gives some helpful perspective. It's of course entirely my impression of your situation based on the info you gave. I could be way off with my assumptions and reading between the lines. Maybe you were a slack and maybe she had good reason to bail. Even if that's the case, I still don't think it's a good idea to rejoin with her. You don't want to change or improve because someone doesn't like the way you are; you want to change or improve because you want to.
The fact that you're "able to see her side of things and why she bailed" is just rationalizing things because you are still attached to her. That's not healthy for you. It puts the frame with her and completely away from you. You're twisting and turning every which way to fit her frame soon. It's very AFC. Think about that.
I'll say again that you'd better detach yourself, cut all contact, make your life the best you want it to be and find someone who's better for you. Cherish the good times you had without deeply desiring them back again. Make new good times with another.
And mind you, it's very extremely likely still the case that she's trying you out again because she has no other prospects. Your assumption that she may be interested in you again because you've changed is probably just that: an assumption. And an incorrect one at that. She's without a doubt using you. Don't let that happen. Move on. She had her chance. She's only in it for herself. She's a selfish, emotional golddigger.
Move on. You don't want to be with someone who doesn't appreciate you for who you are.
