Similar story here. I'm 34, broke up with my girlfriend 6 months ago but let the situation linger for some months. Then I really wanted to get her back but it was too late as she had found another one and didn't want to leave him.
I've noticed that the most painful thing is to keep in touch. It only creates false hope and hinders any recovery. I always found myself feeling much worse after a phone call or email conversation. I've decided and agreed with my ex that we will not be in touch anymore. To me, this is the first and most important thing.
Another thing is: be sad. Let yourself be f***ing depressed and sad. It feels horrible, just horrible. But it's needed. It's terrible to let a loved one go but that's how it should be: terrible. If it would not be like that you would have wasted all the time you were together with your ex.
Don't waste your time trying to forget her altogether. She will always be in your heart one way or the other. Don't worry: you will learn to cherish the memory even though it feels so painful right now.
Don't rush into new relationships. You will find yourself looking for those characteristics in your new girlfriend that you appreciated in your ex. Get a f***buddy OR if you decide to start a new relationship, be DEAD HONEST with your new girlfriend. Respect her: tell her that you are still very sad about your previous relationship and that you want to move slowly. That's only fair as it allows her to draw her own conclusions. Perhaps she's not willing to wait for you to recover. You MUST be honest and open, otherwise there will be no future for the new relationship.
Remember ethic of reprocity: "Do unto others as you would have them do to unto you." Don't wrong others even though you have been wronged. Both bad and good deeds will come back to you.
I wish you all the best, man. Take care. You'll get over it and hey - compared to me you are still young..