How long were you dating before breaking up?
What caused the breakup?
How was your relationship at work after the breakup?
Have you spoken with him since the breakup? (If so how did it go)
Thank you for the additional information, it sounds like your relationship had a lot of drama for the short amount of time that you were together......which is enough to scare most normal men. Men look for drama free women, who click with their life and do not make things harder. When everything becomes a argument and things get nasty often they will avoid any and all contact.
It sounds like he was not very happy with the relationship in the beginning, and assumed when he broke it off that would end it. Then when things continued and you called him, wrote him, etc he tried to run you off by being mean and flirting with the other girl. This is pretty typical for men......and a sign that they are not interested in getting back together.
It sounds like he took some time off, and you should respect that. Calling him now is not going to help your situation. Sometimes a man does not want what he can have.......but wants instead what he can not have. If you are always putting it out there that you want to be back together with him then he is more likely to avoid you.
Instead of chasing him down, ignore him. He has ended the relationship and you have to respect that and move on. If you keep pushing all he is going to do is keep pulling away. Move on, ignore him and show him you could care less about him. If he still has feelings for you then once he has his space he may want to get back together, if not you will have moved on and things will start to look better for you.
I know its difficult when you love someone and want to be with them, but you must respect his choice to end it and move on.