You are saying that the mortgage is being 'renewed', what do you mean by this?
You are reaching the end of the discount or fixed period before moving onto the lenders Standard Variable Rate (SVR).
You mortgage would be for a long term (25 years) and you would be 'tied in' for a short term where you receive a discount or fixed rate.
You are not under any obligation to apply for a new mortgage at the end of this period, and in you circumstances, perhaps delaying any changes would reduce the amount you would have to pay to remove.
Your mother could then apply for a mortgage in her own name in January and you'd have to wait till then to apply with your girlfriend.
HTH
Sparky.