I have been really struggling with one of my ewes this year, she's one of my favourites - one of two ewe lambs I bought six years ago to start my sheepy adventures

- but she has hit a patch of serious illness and I'm beginning to wonder if I'm fighting a losing battle here.
It started with acute mastitis - I took the lambs away from her and treated her with antibiotics and painkillers after much advice from the vet, and despite lots of dire warnings along the lines of "if she's here by Thursday it'll be a miracle" she pulled through.
She now looks about as poor as it's possible for a ewe to look without being actually dead, but I'm feeding her up and she's bright again (though still extremely thin) after a long spell of being very sad. So I'm thinking she may have turned the corner at last with that, although I won't be able to breed from her again and she's started to slough off her udder as the vet warned me she would (yuck and yuck again).
Now the trouble is that she's started to rub her fleece off, originally just her face and now her entire body. The whole field is covered in bits of fleece. I originally thought, head lice, so treated her whole body with Crovect, including direct spray to the head. But if anything it's got worse. She's definitely itchy, even on the bits which are now just bare skin. The fleece seems perfectly normal and healthy, and I can't see any signs of wrigglies in there (though I believe you can't see lice anyway, is that right?)
Any clues? Am I just keeping her going against her own welfare? I know a 'proper' farmer would have shot her weeks ago but every time I resolve to get in the deadstock people she pulls through a little better.
What do you all think? Any thoughts very gratefully received... I really am a bit at a loss as to what to do now.