Try to hang in there!
Obviously you arranged for a vet visit as soon as you saw the gravity of the situation. Worrying about whether the delay will harm her is pointless if there is nothing you can do about it.
So, presumably the horse was in good shape when you left a month prior. That would rule out some kind of long term, systemic problem. If she's lost, what a tenth or more? of her weight, then she's been ill for a significant portion of the time you were gone. That also rules out a very sudden, short lived illness. So look at what could have changed in her care or environment during this last month. Did you board her at this place before you left, or just while you were gone? If she's been there a while, are there any new horses being boarded there which could have brought something (parasite or illness) with them? Is the person caring for you horse someone you trust to not neglect her when they know you are gone?
You probably won't diagnose her problem by considering these things, but you can help narrow down the possibilities for the vet when he or she gets there if you've spent some time thinking them over. It's something to do while you wait, anyhow. I will hold a warm thought for you that everything turns out ok.