Thank you for your letter, I can hear that things have been really difficult for you since you broke up with your ex. I want to let you know that what he is doing is wrong and is a form of harassment. If you are getting unwanted phone calls and text messages from him then you can get in touch with your phone provider (for example O2, Vodafone etc) and ask them to block his number.
If you have a Blackberry or Android phone you might find the free Block'em app useful. It allows you to block certain numbers from calling you, and if you have a Android phone, text messages as well. There is more information about the app in Get involved.
You mentioned that he also has anger problems. If you feel unsafe at any point then you should think about telling a trusted adult or getting in contact with the police. If you do decide to do this it is usually a good idea to keep any text messages and voicemails so you have as proof of what he is doing.
If you would like to talk some more about this then you can always get in touch with ChildLine over the phone by calling 0800 1111 or by logging in for a 1-2-1 chat.