Here are some things that might help:
Parsley, Sage- try drinking tea with these
Decongestants,Antihistamines- sounds like you are trying this already to an extent. Might find out what you can take that won't interact with your allergy meds. I've heard Sudafed will really reduce supply.
For pain and to keep healthy- Vitamin C, Echinacea, Ibuprofen, Lots of rest so you don't get a plugged duct or Mastitis. Call dr or midwife if you have fever, severe pain, redness/swelling, and/or flu-like symptoms. If untreated, you can develop an abscess that will have to be drained and is VERY painful! Raw cabbage in bra is also helpful to relieve the pain.
Try not to squeeze or massage too much in hot shower, because you are stimulating your supply by doing this.
Hope this helps and you feel better soon!
Amanda
Aspiring IBCLC