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She is not entitled to the property any longer. Your agreement was clear and you paid for the mortgage all of these years.
You can tell her she is lost her rights to the property and you are happy to have a court decide but if you have to go to court you will seek an order for her costs as she is the one who wants to breach the contract.
Does that help as a starting point?
You should likely retain a lawyer to threaten a lawsuit.
She abandoned the property and the bank would have closed and she would have lost it and if there was not enough equity the bank could have come after her for the difference.
Now that you save the property and took it on as if it was yours, as per your agreement, she cannot turn around and say she wants it back or wants an interest in it as that was not what was agreed upon and not what was happening the past 5 years. It was yours aside from your not being on title solely for this time.