I have a dust-mite allergy and I've never been sniffle free (only when I lived in Singapore where the air was so dense and dust/pollen and other things didn't fly around - I know they aren't the mites, but I don't know if can have a dust mite allergy and also not be allergic to the actual dust) it seriously sucks but it's just something I've learned to live with.
I'm much the same. I have a severe (off the scale!) dust mite allergy and the only thing that really helped was desensitisation. It was a very long process though and I still have the sniffles most days. I spent around 3 years getting injections for dust mites and grass - one shot in each arm. I started off weekly then fortnightly after a while then monthly and so on. It definitely helped but it's not an easy road.

I currently use Nasonex nasal spray which helps a lot but it's not cheap. I think I pay around $35 after my private health rebate and it last around 2 months, often less if I need to sprays up each nostril daily.

I also take Telfast regularly which also helps. My bedding is all allergen protected and my floors are vacuumed regularly. We are living in our investment property while we build and we had new carpets put in prior to us moving in to save my poor nose. My allergy specialists always recommended floorboards or tiles for me, lol.

If you're happy to try some drugs then children's Claratyne would be worth while. It may pay to see a specialist too. Normal GPs don't seem to know enough about allergies IMO.

Big hugs to your DD. It's a very annoying allergy to have.