A doctor told me that NeilMed sinus rinse can help clear it, if you use it every day. (This assumes the catarrh is originating in your nose or sinuses though. If it’s starting in your throat it won’t do anything.)
Another option is chervil - you can make tea with it. Sounds mad, but it helped my husband.
There are physical causes of catarrh that can be treated - have you seen your GP?
Btw, I didn’t eat dairy for a year because of lactose intolerance. I’m now on pills to counteract the intolerance so I can eat dairy again. My catarrh (and my MS) haven’t been any different before, during or after the year off dairy. Just me of course, might be different for you.