Seeking Advice - When do you decide it's time to euthanize your dog?

My 13-year-old Chihuahua Terrier recently seemed unwell, so I took her to the doctor. She was less active, eating less, and seemed bothered by something in her mouth. The vet initially thought it was a tooth issue, but during a check under anesthesia, they found a growing mass. The vet suspects it could be a tumor and possibly oral cancer. We’re waiting for biopsy results in 9 days.

Since coming home, my dog looks very sad. She hardly eats, only a few tablespoons, and struggles to move more than a few feet. She drinks very little water and spends most of the day lying down. She’s losing weight, and it’s heartbreaking to see her like this. I’ve had her since I was a kid, and I’m scared for her. I don’t know what to do. The idea of putting her down feels awful, but I also feel guilty for keeping her like this. It seems like she’s telling me she’s tired.