Right, you need to pull your head out from the sand and stop being such a nice guy.
Go and see a solicitor. You can play this from 2 fronts -
1) She is missing payments and this is impacting your credit rating.
2) Another man is living in the house.
The child is no concern of yours and it is up to the childs father to place a roof over the head of the child.
Go and see a solicitor and start the ball rolling. Get your solicitor to send her a letter stating she has 3 months to remortgage, if she cant do this then you will go to court for a sale. By giving her 3 months you are being fair and reasonable.
When it gets to court they judge will grant a sale order on the account of, you have no children together, she is missing payments, you have given her time to buy you out.
Of course she is not going to get it sorted when you are not going to do anything about it. Why should she? Stop being so bloody nice and start taking control.