READ THE MORTGAGE! WHAT DOES IT SAY?? DID YOU SIGN IT??
I am so sorry that you are in this situation with your exfiancee. But, to be blunt, that is not the mortgaage company's problem.
What you sign is what they are counting on-- and rightly so. You might contact an attorney to secure payment from your ex, but the Mortgage company has (it sounds like) a valid claim on you and on the house.
It is SOOOOOO Important to understand that what someone says at a point in time is NOT the same as what they (or you!) sign your name to on a legal document.
Hope that helps