My ex has moved on but i still want him back. what should i do?

me and my ex were very close we were together just under a year and did everything together. meals, fun days out, gym we were inseparable. we loved each other very much but argued quite allot. we had split up a few times over stupid arguments but always made up and got back together cause of how much we cared for each other. but this time we had had a bad couple of weeks together not getting on . so when we split up over something silly when we talking about getting back. ( the day we had split) he was with another girl who we both know. and he has been seeing her everyday since we broke up. its torn me apart. i first he said she was just helping him get through it. but then he admitted to me he likes her. and know they are officially a couple. they do everything we use to do together. i see them together all the time as we all go to the same gym and dance studio. he rubs it in my face. kissing and whatever. we was on a talking level trying to be frineds but i had a random txt saying not to speak to him ever again ect. and he detled me off facebook. msn. its only been 6 weeks and its hurting me so bad. why is he doing this? what should i do? and i still want him back i miss him so much?