I'm sorry to hear what your going through, I have no experience in my ex leaving me and thinking I was pregnant but I know about how horrible an ex can be and know what your going through.
My ex dumped me on Halloween and told me over the phone he didn't love me no more and to stop contacting him. I was so upset as we beeb together three years and it was suddenly out of the blue. So the next day I booked a holiday with my friends and then my ex went crazy stalkery on me.
I went out clubbing with people from my new work and he followed me there. He followed me all round my work. And once I got a new partner (my current OH who I'm pregnant with) everything went crazy.
He blackmailed me with sexual pictures saying if I don't go back with him he'll show everyone them, threatening to hurt my OH and turning my family against me. I went to the police and they told me just to change my number.
I went on holiday with my friends and he ruined that too with all the threats creating a fake FB account of me and all that. I come home and my OH told my dad everything what went on and my dad threatened him and he didn't like it so as my OH drove back home he almost crashed as my ex wedged things into his break pads. Now I hear nothing from him and I'm really happy.
What I'm trying to say is don't go back with him, let him walk and stay strong don't cower and hide like I did as it's not good for you. After all this I found put he had been cheating on me and pretending I didn't exist, like some of the other posts had said...are you sure it was a lads weekend away?
I hope you sort it out hun, but don't try and get him back you can do better x