Try another brand of antihistamine, They are not all alike, I have had some that in no way helped my allergy to perfumes that other people use, their soap powder etc. and some that did help.
How good is your vacuum cleaner? Getting cat hairs off carpets and soft furnishings demands a really good one with a good mouthpiece and they are hard to come by.
Are you allergic to cat fur or to the scales on their fur?
And are you allergic to anything else, because if you are, it will make your allergy to your beloved puss worse.
Keep a diary of what you eat and drink, wear (natural fibres contra synthetic as either category can be a cause of an allergy) stop using any bath products, laundry soap, cleaning agents etc. that contain perfume and check that none of it contains formaldehyde. (perfumes and other additives are likewise frequent triggers for allergies).
Wear a surgical mask when hoovering, dusting etc.