I ran into the same thing initially around 3 months, when my period returned, and used fenugreek after that during periods, which helped some, but around 9 months I hit the pumping wall. I changed membranes, etc. but didn't see improvement (this doesn't mean you won't! definitely worth trying), and at 10 months-ish wasn't able to produce anything with the pump anymore. What worked for us was that my husband started bringing my son to my office at the time I would normally pump. My supply was fine, but I didn't let down for the pump anymore, so this worked well. Is going to your daughter's nursery possible, even for a lunchtime nursing session?
Also, I don't know how long you're planning to pump/nurse, but even though I stopped pumping around 10 months, I was able to nurse until my son was 21 months old, so not being able to pump doesn't mean you won't be able to nurse if you want to (I was really worried about that).