The breeder from whom I bought Smokey 10 years ago told me she had a situation like this, where one young neutered female Siamese was being mercilessly bullied, and was crawling around submissively on her belly all the time. In the end, she gave her to someone else, to be the only cat in the household, and she became much more confident and happy. Maybe this is the only solution, and it's very sad for you I know. But it's not 'giving up', just doing the best for the cat. I suppose I've been lucky over the years, and been able to let things settle of their own accord.