Little Freesia, I'm here to offer you loads of hope, as I could have written your post a few years ago. My twin2 had lots of feeding problems, hole in the heart, hydrocephalus, was off the centiles, had very severe reflux, never slept and cried 14 hours day. I thought it would never end.
To be honest, it IS tough, very tough when one baby doesn't like eating. But this phase will go. My son was very weak until he started school. he began reception in age 3 trousers which were taken up 6 inches because he was so short (they didn't do age 2 school trousers.) He was bald as a newborn too. I felt so inadequate to help him. Now he's 8 and in age 10 trousers, bursting with life and health and cute and funny. Having been bottom of the class for the first couple of years, he's now near the top.
I'm sure all this sounds like it's not applicable to you in your circumstances but I just want to reassure you, it can change. It can get brilliant. She will one day eat and enjoy food. She will one day catch up. You will be a normal, happy family. It's just that with twins, when they're delicate, the first years are very different from what you anticipate. If you're merely coping, you're a supermum. If you're not coping, that;s understandable in the circumstances. You sound like a great mum who's doing a great job.
You mention not being able to enjoy them. We had this problem. The best solutions for us were:
Take them out one on one. I couldn't believe how easy it was just to have one baby. I remember a day on Oxford Street feeling like the smuggest, best mum in the world, because my one baby was such a peach to manage, after trying to cope with two every day.
Also, we got out of the house a lot. I'd wrap them up in cosy blankets and walk along the river or in parks for hours. The buggy seemed to soothe them. Physical activities were limited - but we went to a music group, as much for me as for them, and we all loved it.
Also, I didn't care what sort of day we'd had, I made it a mission to get us all giggling at least 3 times a day, and to play with them for half an hour before tea. Sometimes it was just going through the emotions of enjoying them - probably a lot of the time, that's what is was, but it means I do have good memories, and they had chuckles and we can chat about what we used to do when they were small. Sort of bank of good memories. It was an effort, but worth it. My strongest memories of the early days are from when we took the babies out and did things with them, individually or together.
Convince yourself it will get easier. Convince yourself there are loads of bright and gorgeous times are ahead. Accept it's tough and horrible now and give yourselves as many small breaks and treats as you can.
One thing I can say is, once the tough times began to pass, we seemed to enjoy ordinary everyday living more than anyone else I know, and that's never stopped. Having a tough start means you truly appreciate it when the load lessens.
Meantime, I hope you get lots of help. Try not to neglect yourself. I did - I stopped eating and got very stressed. If I had that time again I'd take really good care of my own health and moods. Get support wherever you can. Try to have a night out once a week (not easy I know.) Take time to find what you do best as a family - we lived outdoors as I hated being stuck at home, but another friend lived in PJs for two years with CBeebies on full blast. Doesn't matter. Whatever keeps you calm. You're the one who needs to keep her energy reserves in place. Whatever works for you is fine. Don't let anyone ever judge you or tell you how to do it better, not HVs or docs or anyone, if they haven't been in your position. They don't know. Textbook advice doesn't ake any provision for sickly twins.
Do pester docs for medication. Is she on Gaviscon? Ranitidine? Zantac? They helped us so much.
Another thing I did was give up on meal times. They took hours and were so stressful. It wasn't ideal to do this - my two still don't have great table manners, compared with the rest of their manners. But it was so much more relaxing to just chop up lots of healthy bite-sized food once a day and then post it into his mouth when he was distracted. He ate far more that way, than if we sat and tried to give food at one sitting. One thing i do wish is that I'd given more attention at food time to twin 1 who ate well.
Sending you lots of love and support and admiration. I know how it feels. It was the toughest time of my life but we came through happier, stronger, and I'm sure you will too.