for my dog

so i have a dog for 12 whole years who basically has sticked with me for my whole childhood, and recently she was been acting weird. my mom quickly went to the vet who checked on the dog. everything seemed fine, but days later the results came and we found out she had cancer. she has to get a surgery but that costs really much. basically we can't really afford it. the vet said we have 4 months left and if she doesn't have the surgery, we all know what will happen.