Sorry to hear about this Slimline, but take good advice of what someone said previous and that is do not dwell on the fact that there may be someone else. Going through this a few times personally (and I'm not very good at relationships post-facto) I've found myself dwelling on that possibility. I'm not sure about your case because each is unique, but for me, it just seemed like a good way to cop out in finding a reason for "why". I've pretty much come to realize that there may not be an answer as to "why it happened". She can tell you one thing, but most likely it's such a complicated complexion of various different things that ultimately caused the breakup. In the beginning, it IS impossible to try to stop thinking about her, but it WILL get better as time goes on. Try to do something else in your life, go out with some friends, try something new. What worked for me mostly was that I went to the local gym and played a lot of basketball etc etc. Know that your friends are there for you when you need them.
One small piece of advise which may or may not concern you. I had a problem where my ex was good friends with pretty much most of my friends as well (high school sweetheart), and I put my friends in a tough place where they had to decide between me and my ex. Try not to do that, realize that your friends are probably in a very precarious situation as well. Best of luck on moving on and finding that someone on down the road. (It WILL happen)
One small piece of advise which may or may not concern you. I had a problem where my ex was good friends with pretty much most of my friends as well (high school sweetheart), and I put my friends in a tough place where they had to decide between me and my ex. Try not to do that, realize that your friends are probably in a very precarious situation as well. Best of luck on moving on and finding that someone on down the road. (It WILL happen)