Hey man,
We've all been there at some point.
Every article you'll read on "How to get your Ex back" will say the same... you get her back by moving on.
And by the time you move on, you probably won't even want her anyway.
Sounds counter intuitive doesn't it? "But if I just said/did this....!" Don't!
If she broke it off with you, no gushing or promising or attention will make her want you back. What'll get to her though is seeing how you just move on, meet other girls, other friends, look active and happy. Fake it for a bit if this is still raw in your mind.
At that stage... what happens?
If she still wants you, she'll come back.
If she doesnt, she won't.. but you'll have moved on. You can't make her come back.
If she DOES come back... and you're truely getting on with your life... it might feel right to give it another shot, but often, it'll feel like going in reverse but that stage and you won't even want to. Doesn't feel like it now but it's true.