Why do I still think about my ex-girlfriend over a year after the break-up and can't seem to really move on?

Hi all. Why do I still think about my ex-girlfriend over a year after the break-up and can't seem to really move on? She won't let me contact her. She changed her cell number on me (she told me after she did it), deleted and then later on blocked me on Facebook, sent me back a "Return to Sender" snailmail letter. I want to move on with my life in a healthy way. I just feel that she was the one for me and a very special woman to me even though she didn't think so obviously. I don't want to sound creepy but I really don't know how to get out of this overall mental situation. She's clearly moved on with her life without me and she has lived with at least one other guy that I know of. Her deciding to completely cut me off out of her life after the break-up has been devastating. I miss her everyday. It's destroying my life in many ways. I haven't seen her in almost a year and yet I still think about the 6 months of good memories her and I shared together when her and I dated. Her and I are about 30 years old by the way. We're not just kids. Any good advice would be appreciated. Thanks.