From my own experience, allergies can make you feel like you’re sick (tired and miserable). Personally, I only feel itchiness when I have allergies, so when I feel itchiness on top of those other symptoms I know straightaway I’m having bad allergies. If you know that you get to feeling this way and having sinus infections at the same time every year, I’d agree with your guess that it’s seasonal allergies.
I’ve never had one, so I don’t know the specifics, but you can get an allergy test to see what specifically you might be allergic to.
From my own understanding, not all antihistamines work as effectively for everyone. I used to knock out all of my symptoms with a certain antihistamine. I’ve used it since I was ten for pollen and cat allergies, but over time it’s stopped being as effective. After going through some experimentation with other antihistamines, I finally found one that works for me again, and it’s a world of difference for me. I can walk past grasses once again without feeling on edge or needing a box of tissues in tow. (I also found two more antihistamines that don’t work for me—on top of one I already knew from years ago— but they work for members of my family.)
So, I’ve got one drug that works for me right now, one that used to work but no longer does, and three that I know don’t help me—and that’s just within the over-the-counter non-drowsy options.
Again, just my experience. I don’t know how many antihistamines you’ve tried. And I’m certainly not any sort of doctor or knowledgeable expert on this. I’m just speaking from my own experience.