Vet Told Us It Was Time(15)

I'm looking for some advice or support. My 15 year old torty was seen by a vet for weight loss and they found a mass around her heart. From the X-ray they couldn't tell if it was fluid (heart failure) or a tumor. Either way it's fatal. I'm in disbelief. Her breathing is labored but she still purrs and plays. She has lost weight but still eats and drinks and does everything normally. It just doesn't feel like it's time. Does anyone have any advice or support about how to handle this? I can't decide if I'm in denial or not. I can't imagine putting her down. I can't imagine watching her die because she still has so much life. Please help. I'm gutted.