that's always really hard, I'm sorry for you *hugs*
The thing is, seeing our ex with someone else, or hearing about it is always going to hurt. It's a direct comparisson between you and her that you wont be able to help making: "is she prettier than me?" "does he like her more than he liked me?" etc...
We might think we can deal with the idea of them finding someone new until it actually happens, and there's no need to feel bad that this effected you so much. It will always be a shock at first, but I think you'll find it wont last too long. You'll feel jealous, naturally, and as if your role at his side has been "stolen" by her, you may want to hurt them and/or feel anger at them both. I think all of this is probably quite natural and by writing about your problem here you are finding an outlet for your pain - the old cliche of a problem shared being one halved is usually true

talking to other people, writing about it,and doing anything you feel helps you let the pain out (except hurting them or yourself) is probably a good idea - as human's we need a release for pain and I find language and words are mine.
There is someone I will always be in love with and now I've learnt to accept he'll always be special, it hasn't hurt in a very long time, and I don't miss him, but when he first met someone else I was devestated. I compared myself to her constantly, and the thought of them together was the most painful thing that had ever happened to me. Even recently, hearing they were engaged made me a little sad again - it's never nice to hear the love of your life has found his - but the point was it only made me a little sad, not totally in pain. Time will do this, eventually.
When they first were together - say the first six months, I tried to stay out of the same room as them and I might advise you to do the same - I wouldnt want you to go through what I did, watching them kissing on the row of seats in front of you

out of sight out of mind is another cliche that's at least partly true - the more you see them together while things are still raw the more damage it will do to you. Don't obsess over avoiding them, but if you have the option to walk out of an area they are in and it's hurting you to be there, do so. There's no point staying and torturing yourself.