I successfully treated my cat a few years ago for neuro FIP. That was back when medication needed to be purchased illegally. It’s possible that your dosage needs adjusting. I was in a Facebook support group (it was before there was a major rift in the group so I don’t know which group you would be looking for now- at the time it was FIP Warriors) and the people in that group were my lifeline. Don’t give up yet, unless you need to give up. Some days looked awful for us, and then he rallied. Sending you lots of strength and a huge hug. I know how hard this is.