I can see you are in a difficult situation and it's hard to know how to advise you. I think it might be a good idea to discuss what has happened with your Social Worker.
It might also be a good idea to contact your landlord and explain that you are now a sole tenant and see if you can get the tenancy agreement changed. My son split with his girlfriend very recently and that is what he did, he can now claim for some help with his rent and council tax, now that the tenancy is in his name only.
You could try going to your local Citizens Advice Bureau foradvice and support. If you are having problems with your landlord Shelter might be able to,give you some advice and support. Here's a link to their website
I can't advise you on whether your girlfriend will return, pregnant women are often very changeable and can have mood swings, I would advise that you try to be understanding of this and make allowances for her right now.