Ex-girlfriend not paying her share of mortgage

[JD1974]

I bought flat with a joint tenancy mortgage with what is now my ex-girlfriend 4 years ago. About a year and a half ago we split up and she moved out. I rented out the spare room and she topped up the mortgage and bills on a monthly basis as the rent didn’t fully cover her share of the mortgage. That was until 6 months ago, when she stopped paying her share. I understand that she has some money problems and is unable to pay me back what she owes or continue paying again. I have been covering the mortgage payments until now as I did not want to default. I have looked into buying her out but as my salary (including the rent) is less than 4 times the value left on the mortgage I was declined. I do not wish to sell the flat and understand that she cannot force a sale. Is there anyway I can prove that as I am paying the mortgage, not her, I have a greater entitlement to any equity should I decide to sell the flat at some point in the future ?

[ilovetohelp13]

Why bother her with payments when you guys have split and she had moved out? The place is pretty much useless for her. She should not have paid anything at all.