I really feel for you, because it is obvious you and she are so bonded and you love her very much.
But you must put her welfare higher than your own fears.
Your girl urgently needs medical treatment. She may even need surgery. Even a mammary (breast) cancer -if it IS that, can often be cured by surgery alone. But that will not be cheap, and is likely to be a few hundred dollars/pounds etc.
Obviously I can't diagnose, and I don't know if that is what is wrong with her, but there may well be a solution to her health problems.
Now you might not have a lot of money for this, butit's up to you to find a way. If for instance you have a bank account, you may be able to go to the manager of that bank to arrange an overdraft to cover the costs of the vet's treatment or any surgery.
That would be the quickest way -or putting it on credit card if you have one and paying that off monthly.
But the most important thing is getting her seen by a vet and assessed and any treatment started as soon as you can.
I know how awful it is -to have in the back of your mind, the awareness that one day she will have to leave this world, and you feel you cannot bear it.
But don't let that fear hold you back from helping her now.
I felt exactly the same about my own dog. She eventually died age nearly 15. She was a Lab mix. I thought I couldn't live without her (and still do miss her to this day) But she soon showed me after she'd left....that her Soul and her love never ever really did leave me, and that she continues to love me. That was the most amazing thing to find out. They live on and always love us.
So do try to be brave, for her sake. Get her some medical help as soon as you can.
Blessings to your dog! And to you. I love anyone who loves a dog the way you do!