This is a tough one. I am not a professional, nor am I talking from experience really as I don’t have that problem myself, but maybe this makes me a good person to talk about it. I don’t know. Will try to give my 2 cents in hope that it helps.
I don’t want to say much about your specific case as I’m sure people already told you you should just forget about him and move on, but a small note on the situation. To paraphrase a poem by Emily Dickinson:

Love is a bee.
    It has a song –
It has a sting –
    Ah, too, it has a wing.

Now this paraphrase rings false, and my question is why. Why we don’t think of love in that way, but rather that it is eternal. I think it is eternal and whether you break up or not you both probably still love each other, but then life goes on and one continues to live. You shouldn’t think it means much.
For instance, people deal differently with being alone. Some people can’t be alone for a day, and would immediately find someone else. It doesn’t mean they cared less about who they were with. (People, as you know, break up from different reasons, and if falling out of love was the main reason for breaking up you would see a lot more breakups as sadly it is rarely what keeps couples together).
To summarize, him already dating says more about his character than about his relation to you.  

But now to the topic of anger management.
No counting to 10 here. Seneca says that anger comes from the feeling of weakness. I would recommend listening to Nina Simone’s Mississippi goddam, or her version of Jenny the Pirate (in for instance her 4 disc compilation: Four Women) . That’s anger for you, the anger and ferocity of the weak, but strong. Why are you weak - because of time. You can’t turn the clock backwards. Everything you did for him in the past, everything you said, the time passing, all happened and you can’t do a damn thing about it. That pain creates an amazing amount of anger that simply can’t be released. A pain which, having nowhere to go, augments itself as sound in an echo chamber.

Descriptions are cheap, what do you actually do about it. Well, descriptions are cheap, but understanding the origin is golden. It is the difference between going to the doctor and her telling you a description of everything that hurts, and her saying what is the disease. Diagnosis is not yet a plan for a cure but it’s a start.
I’ll point out that descriptions are what most advice books/columns are quite good at. They can very accurately describe the case and what you are feeling, but then their diagnosis, their analysis of the situation, is usually complete garbage, as are their suggestions. Usually, I’m sure there are good ones too.

Now, what do you do about it ? Well, hopefully, understanding the problem – that you can’t go back in time – will help alleviate some of the anxiety from it. It is simply the case and you can’t do anything about it, so chillout. Now, this makes you angry – chill out – yes one doesn’t like hearing that as it is exactly what you can’t do. There’s nothing to put out the fire with. For Freud, and I guess nowadays psychology, if you experienced a trauma, bringing it to the surface, to consciousness is a big part of the cure. So yes, try to figure out exactly what are you upset about, in details, and then that it is the past.  You’ll move on, and time here is also a great cure. (Well, I wouldn’t say a cure already as many meet ex’s decades later and start yelling at them, and many times time just buries it in the back in all its might).
The moment you won’t feel you need to react to it, that you need to be strong and respond, the moment you’ll just accept it as the past and move on, while learning what you can from it, you will feel much better. Of course, learning from it is the biggie, but it also helps to direct the anger to something useful, which renders it weak.

This is, of course, just a certain aspect of it, but it’s a start.
Hope this helps a bit.

Music Discussed

Dear Amanda,

I wouldn't go so far as to say you "shouldn't care," but nonetheless you have put your finger on the key point: if this emotion shouldn't "rationally" result just from his action, where exactly does it come from? Perhaps you feel that you have been deeply humiliated; perhaps it is some variant of this, e.g. you are horrified by what people will think.... 

As others have said, you'll need to understand much more deeply what is going on before you can let it go. Emotions don't respond well to rational bulldozing. As a practical matter, you might try asking yourself a series of extreme questions, depending on the situation:

How would I feel if he got back together with an old girlfriend instead of picking up this new girl?
(or: How would I feel if the person he were dating were someone completely new rather than his former fiancee?)
How would I feel if the new girlfriend were less attractive/dumber/obviously less interesting than me?
How would I feel if he moved to Australia and started dating someone new there, and none of my friends knew except me?
How would I feel if I knew she really didn't care about him?

You get the idea. It is as if you are trying to determine a physical injury. Why exactly does your leg hurt? Which movements are painful and which are not? Try to exaggerate the different possible aspects of the situation; some may be very tender, while others won't move you at all.  When you finally find the locus of your pain, you will need to be very careful -- and also very understanding -- in trying to unravel the knot further. What precisely is the source of this wound?

Once you have done this -- addressing the true source of anger is difficult in itself. But it is a much more rewarding and rational process. Best of luck.

Dear Amanda,
A bad relationship leaves you in a bad place.  The worse a relationship was, the harder it is to get over it.  Your number one priority is to get back on your feet.  Certain questions, such as why the heck did I date this person? linger.  Also, how the heck can I do better next time?  You wouldn't feel angry if you didn't feel out of control.

You have to avoid thoughts about warning the new girlfriend about the faults of your ex-boyfriend.  This is where time helps, because if you imagine her asking you for advice, you can respond, "Oh, but it was such a long time ago, I don't know what he is like now."  And to yourself, you say, "Surely he has grown up by now."