Obviously I have no idea you or what kind of person you're. I have no idea how you taken care of your ex or when the breakup was mostly your fault or hers. But what I do know is that generally there are a few common reasons that relationships fall apart and knowing the complexities, and the remedies, can go quite a distance to making sure you can get back together once again.
In most situations people have one or both these issues within their relationship: a lack of respect for their partner and a lack of communication. Figuring out which one you have problems with and finding methods for getting over those troubles can go quite a distance to making you a better individual and that can go quite a distance to making your ex partner want to be with you once again.
When people complain about their human relationships among the first issues I inquire further is: do you still deal with and talk to your partner how you did when you initially met? For instance, are you polite? When you wish something from your partner can you ask them for this and say make sure you or can you 'order' them and neglect to say make sure you or thank you? I'm occasionally appalled at just how I hear young couples talk to each other. If you're more polite to the one who gets your coffee than you're to your partner, you have to question why. Why would you be more polite to a stranger than to your partner? Kind of strange isn't it?
So, start there. Take a very long hard go through the actual method you talked to and treated your ex partner. Try to see what areas you can make some improvements in and make them. I'm not saying that you are the only person that must make changes, but for now you can only change you. In the event that you and your ex get back together hopefully she'll face her own issues and make whatever changes she needs to make, but for now you worry about you.
Common problem two is that many couples don't know how to talk to one another constructively. They will argue and bicker and feel resentment because their companion doesn't 'get' them however they don't really connect. Often we simply aren't proficient at asking for what we wish. We somehow anticipate our companions to simply 'understand' what we have been trying to say. Newsflash, most of us aren't mind readers so if you (as well as your ex lover) can't learn to express yourselves within an clear to see, non accusatory method you'll never get anywhere. Learn how to politely and respectfully require what I Lost Love-Can Be Pain Of DAYS GONE BY want. And encourage your lover to do the same.
For all you guys who are saying "I skip my sweetheart and I'd like her back" these suggestions can really help. Not only will you be helped because of it turn out to be someone she would desire to be with, it will help you become a much better guy which is always a good thing.