Thank you so much for everybody that posted.
Your kind words helped me so much.
I still feel so sad and empty inside. I wonder around my house and I just want to sleep.
My brain has told me that he is not the one for me. That I deserve better, but my heart still hurt so much.
The worst thing is that he has tried to turn the tables. He has changed his relationship status to 'Widowed' on his Facebook profile and told me that I have been sending messages on Facebook to people all along. This is not true.

I don't know a more faithful person than myself. We sent each other some emails after and had one telephone conversation - All this happened on Friday.
I have now cut all contact and try to forget about him. But it still hurts so much and I am so angry with him.
He started to accuse me of cheating and that I flirt with colleagues in Friday's email / call. I don't even know where this has come from. He has never ever mentioned this at all during the relationship. It is not true.
I know that it should not bother me, but I know that he probably received lots of messages on FB regarding his status change (I am no longer his FB friend and can't see his wall) and they all will see him as the victim. As we don't have any FB friends in common, nobody will ever know the truth. He is playing the victim and everybody will believe him.
Another thing that hurst me so much is that only 4 days after this has happened, he updated his Plenty of Fish profile and is looking for 'Long term'. He also is looking for a girl that loves 'looking at the stars with your arm around your loved one'. I took him out on a date a week or two ago. We went to the park at night and looked at the stars with our arms around each other. He said that he hasn't done that ever. Now it is on his profile.
How could he do this.
What hurts the most is that he isn't hurt by this. He doesn't miss me. He is just going on with his life and is flirting online as we speak and probably arranging dates.
I wish he could feel the hurt that I am feeling.

I don't think that we were meant to be together and everything wasn't going that great. But the betrayal hurts so much. Why didn't he just left me (or why, oh why, didn't I leave him!).
I wish that we had a lot of friends in common, so that somebody out there can tell him that he is wrong! But as we only met 4 or 5 months ago, and we met online, we don't have many friends in common. He met my friends and I met his friends, but his friends were not my friends yet.
I wish I can give you ladies his Plenty of Fish profile name to go do a bashing... but I also know that, that is the anger inside me talking. I know that I will regret such a thing later.
Oh, the pain, please go away.