Though right now you probably don't even believe it's possible, you definitely can improve your chances of getting your ex girlfriend back and be successful - it is not a hopeless situation.
But first and foremost you've got to get your head straight.
Really be serious with yourself and look in the mirror and examine your actions right now.
Are you being the type of person that would attract someone to you? Or are you trying to guilt her and manipulate her, and change her feelings and get her to understand you, etc...
None of that will do anything but drive her further away.
  Step 1 - Get yourself turned around.
Pretend that you have a once in a lifetime opportunity to start dating someone that has been your dream girl forever.
What would you do? How would you act? Would you be strong, confident, dress sharply, maybe get back in the gym and pay a little more attention to your behavior? I think so.
So quit focusing on the negative of "I've lost her".
Start focusing on what you've got to become to get her back - and do it.
  Step 2 - Stop pursuing her.
That's right.
Focus on being happy, being content with the separation, focus on your work or a hobby, whatever, but stop chasing her.
Let her see your changes (don't tell her you've changed - let her discover it for herself) and let her seek you out.
  Step 3 - Start dating again.
Now I know you're going to tell me that this is the only girl for you and that you have no intention of seeing anyone else.
Listen, I'm not saying you have to have the intention of falling in love with someone else, but you do need to see other people.
It is important that your ex sees that you're not just "hanging around" waiting for her.
Her response to you seeing other people will be dramatic - rest assured.
  So there you go with three quick tips on getting your ex girlfriend back.
Now, once you get her, don't be dumb and repeat the same mistakes that caused you to lose her in the first place!