If you want to get her back then I bet you've been sending her expensive gifts, trying desperately to get her attention and prove something to her about how you feel. Chances are none of that has worked, because to be honest it's pretty old hat and your girlfriend wants to see something more honest and much more innovative from you.
A dumped girlfriend is likely out to make you jump through a few hoops before she takes you back, even if she desperately wants you back. And if you did dump her, it's going to be hard to argue against her feeling that way.
You've without a doubt greatly hurt her feelings and her pride and she's going to exact some revenge for that. Depending on how desperate you are to get her back will tell you the level to which you'll be prepared to put up with what she throws at you.
If she needs you to explain what you did and why you did it, then you better take some time to figure it out in your own head and be sincere when you sit down to explain it to her.
If she needs an apology from you and a sound promise from you that you will never do it again, then you're going to have to do that.
Maybe you panicked as you saw the relationship deepening and getting more and more serious. It's a pretty common feature for some guys and to suddenly feel as they are feeling smothered and overwhelmed in a relationship and instead of sitting down with their girlfriends, they panic. Before you know it, they have a dumped girlfriend and they're living with regret. So perhaps this is the situation that you have to explain to your ex girlfriend. If it is, again, be honest.
What you must guard against in a situation where you were clearly in the wrong and you over reacted, is that you don't become her punching bag. Don't allow her to use you as an emotional vessel into which she pours out all her frustrations and anger just because she can. So doormat out, understanding and apologetic guy in!
Chances are if the two of you still have feelings for each other, you will get her back after you dumped girlfriend, but you have to be clear that you're happy to try again and not simply reacting to feeling bad about the way you treated her.
Read more in The Magic Of Making Up