I have a feeling I am losing my almost 3 week old preemie bernese mountain dog puppy. She was born weighing only 5oz and

Customer: I have a feeling I am losing my almost 3 week old preemie bernese mountain dog puppy. She was born weighing only 5oz and at day 3/4 mom rejected her completely. I had to step in and since then it's been every 2-3 hrs of bottle feeding 24hrs a day. I keep her in a warm incubator and at day 9 she got so weak and kept spitting up milk so I had to start tube feeding her. I tube fed for 5 days straight, and a lot of touch and go moments. Day 14 she was strong enough and took a bottle again for the first time since being tubed. She has always pooped and peed very good throughout this time. She was doing great, eating sometimes too fast so I changed the bottle to a slower flow, but she had energy and fight to her. Yesterday I took her to the vet (yesterday Oct 22nd) because she wasn't pooping everytime she ate and then got very bloated and stayed that way. They tested her stool for worms and she was clear but they did give her an enema and she pooped a lot. He said she was backed up. So he sent me home with another enema in case it continues. Things took a turn for the worst last night and she started foaming out the mouth and wouldn't eat. This lasted all night long, she struggles to eat and breath and is back to not wanting to poop. She wakes up crying sometimes like she's in pain. She weighs 18.50oz now and is 19 days old today (23rd). I thought she was out of the woods but she truly is not. I gave her some karo syrup today with feedings and she perked up better, but is still struggling to breath sometimes and has foamed out the mouth about 20min after every feeding. I'm at a loss, the vet said she will either live or die and I am trying my hardest to keep her alive. I've been to the vet several times and not much they can do for her except euthanize her. That is a complete last resort. I don't know what changed she was doing great until we visited the vet yesterday and got the enema. Do you have any suggestions as I feel she is slipping away and I don't know why???
JA: A lack of appetite can seem worrying. The Expert will know how to help. What's the puppy's name?
Customer: Olive
JA: Is there anything else the Vet should know before I connect you? Rest assured that they'll be able to help you.
Customer: There was only a litter of 3. One was still born and the other died at 2 weeks because of spinal injuries from mom. She is the last one surviving