Only one solution: Get rid of the cat. No amount of washing, de-fleaing, hoovering will make a blind bit of difference – you’re allergic to a protein in the dander and saliva. The filthy little beasts spend all day licking themselves so dry saliva dust is constantly falling off them into the air.
Hoovering and dusting just sends clouds of the stuff into the air making the problem worse.
You can take antihistamines every day if you don’t need to work, think, speak, interact with others, socialise etc. because no matter what the label says, antihistamines make you drowsy.
The ones that claim not to are a con – they basically make you drowsy but then give you a shot of pseudoephedrine to try and wake you back up. This is awesome because it adds insomnia to your list of woes.
Trust me, the cat must go. It’s the only possible solution.