ER Vets are generally not the best, especially the overnight ones.
I would recommend finding a good vet to take her to. Your situation isn't uncommon and there are a lot of vets who are more than willing to work out payment plans if they know the other options are 1) the animal doesn't get treated or 2) they treat the animal and you never pay them. There are still a lot of vets who do it for the right reason (wanting to help animals) but they still have to get their costs covered eventually.
Hang in there. Remember, even if she were to die today, she's lived a better life with you than 99% of the animals out there, including humans. Sick pets are hard, for a long time I wasn't able to bring myself to have pets because of how hard it is to deal with them being sick or dying. You just have to remember that she's made you happy and she's had a good life.
All that being said, when we had Mr. Piggy, there was an overnight ER doc that tried to get us to put him down ,and he ended up living a long, fruitful (well, as fruitful as a guinea pig can be) life after that.