As shep53 says, either keep her in the barn on hay (not out on grass) and it will dry up, or get her a lamb. She'll need company though, so do you have another sheep you can keep in with her? Or maybe a patch of very poor grass you can put her on, where you could feed her companion but not her?
It's much better for her to get her a lamb if you can. It sounds like she's not a first timer, so she will be grieving and will love to have a replacement lamb. You may need to hold her to let it feed for a few feeds, but IME bereaved experienced mums take a foster very readily.
Do check she has
got milk first, though!
And don't beat yourself up.

Lots of people lamb outside and for the most part the foxes would only get lambs that were dead or dying anyway. Lambing indoors brings its own risks and losses.