As it is your horse is on the bottom of the totem pole, and I say she is going to stay there. Some horse's will stand up for themselves, others won't. Like your mare, my gelding is attacked and bullies by everything else with 4 legs and a tail - even kid's ponies bully him!
For the sake of €50, you'll get a heated indoor - if you feel you can afford it, go for it. I'd be loading my horse into the horsebox and be gone. If she's a timid horse, she'll keep getting bullied. I say being an individual pen or field with other timid horses may suit her better as obviously somebody in her current herd is quite the bully and there is a very obvious personality clash.
Stop for a minute, open your mind, learn. You may not agree with what I say, I may not agree with what you say but we will both learn something new.