You need to put her on a diet of straw + minerals, or, as you rightly say, her chances of getting in lamb this year are small.
Obviously you dont want to keep her on her own, although, as a last straw, this may be preferable to the alternative. But it cannot be unsurmountable to work out some way of giving her a companion, even if it means feeding the companion extra feed each day in a way that the overweight ewe can't get to it.