I wouldn't say to let things go and she'll come back to you if it's meant to be...that's a bunch of movie bullshit.
Women remember things for a loooooong time. And I promise, you're not the first to do her wrong.
Breaking up wasn't too good of an idea if you wanted to be with her...I could understand you not feeling like she cares but she was with you right? She might have been waiting to see if you were just being nice to get her back, but then start acting up once she starts caring again...it takes a lot to prove to a woman that you've changed after you've done her wrong. Either dig your heels in and be ready to make it up to her, or let it go!
There are a lot of guys who say the nice things after they've hurt you...and as soon as the girl gives in and let things go back to normal, the guy is back to acting they're normal abusive / disrespectful selves! You breaking up with her probably showed her all she needed to know...you weren't willing to do what it take to prove to her / heal the wounds! Even in the animal kingdom the female wants to be pursued! The males that stick at it the most and fight for her no matter what are the ones who get her. Why do you think women play hard to get? They know you really want them if you'll even jump through hoops for them.
If you really want to be with her, you'll explain why you broke up, apologize, and spend your time contemplating on how many different ways you can show her that you want her and will treat her right. If you're not willing, another guy will step up!