Hi
I am very sorry.
Unfortunately, the thing about mammary tumors is that they just continue to grow and to grow and to grow until the legs can no longer touch the floor and then they continue to grow.
Mammary tumors can be both benign or malign but are generally not too bad to operate out as long as they are still encapsulated.
It means that you have to either face a make or break operation or euthanasia . It is unfortunately one of these horrible 'between a rock and a hard place' situations where there is no easy way out and the choice is between two rather difficult options. In terms of an operation, it all hinges on your vet's confidence and practical experience in operating on small furries. I know of a number of mammary tumor ops that have come off well, even where the tumor was already quite large.
I REALLY wish I had better news; especially at this time of year. At least you can be proud that your girl has lived to a good age.