We all medicate with second-gen antihistamines year round. IMMV, but honestly, if you even suspect that this is an issue, it's time to hit the doctor's office, and I mean a board-certified allergist. First step is going to be to find out WHAT allergens are an issue (if any) and the next is what to do about it. This is not really a DIY project, in spite of popular sentiment on that subject. {sigh} The problem is correlation fallacy-- we're all susceptible to both that and to placebo effects when we try interventions on our own.
IF you see allergies that are having this kind of impact on function, it means (at a minimum) lifestyle modifications. So that (IMO) calls for professional advice.
Atopic disease can be quite serious and have long-term health impact if left untreated/unmanaged. Asthma is a frequent underlying component of atopic disease.
I know that a lot of people swear by supplements or honey-- but mechanistically/scientifically, there's really nothing to suggest that they do (or can) work. Aeroallergens aren't really the type of pollen that bees collect-- if they were, they would rely on insect pollinators, and in that case, they wouldn't be wind-dispersed.
At any rate, that PSA aside, out of control allergies generally lead me to feel stupid and extremely irritable. There is at least one study that shows that atopic people perform FAR worse on A-levels than those without seasonal allergies. So it's a real effect. Medicating with sedating antihistamines can obviously impair cognitive function substantially, too.
