I'm not in this situation, so what I'm about to say may be easier said than done. I think in your case you really need to be a great observer of your child (all parents need to be, of course) and know where your child is and what she can do now. Because 'before' was just that, before. Things will change, your child will change, and your expectations will need to change, too. It's just they are going backwards instead of forwards.
For her, so that there are expectations, perhaps you could bring it to things like being honest, being a good friend (all ages and all developmental levels/stages are capable of being friends, and in my imagination wouldn't go "backwards"but hey, I have no idea so school me if it's not true). being thoughtful, caring, loving..... that's what I would focus on, not whether she could feed herself before but not anymore, walk a long time and now not so much, etc. (or whatever the issues are)
And I'm sorry she and you are suffering. I can't imagine how hard this must be.