A mental illness isn´t static. Just because Nevill had been able to calm her down in the past, there is no guarantee there wouldn´t come a point where he couldn´t.
Those in Sheilas' case,sadly get worse as time goes on because of on-going issues in their lives with which they can't cope with. Anything can " tip them over the edge ". Those issues that WE can solve,they can't,and so their condition worsens.
As was plainly and clearly stated,Sheilas' condition had deteriorated. Deterioration in a patient,is NOT a good sign,regardless of what illness they suffer.
Deterioration in anyones understanding means WORSENING ! Sheila was very sick !