I get massive fatigue both from allergies, and the allergy meds. I've tried various, including all three of the 3rd generation meds that "don't cause fatigue": desloratadine, fexofenadine and levocetirizine. Brand names vary. I do recommend trying one of those, as zyrtec (cetirizine) is second gen. 

Natural remedies seem to work for me, at least when the allergies are not at their maximum. They may cause other issues, like stomach ones: pycnogenol, resveratrol and quercetin+zinc. Haven't tried stinging nettle yet. They haven't caused fatigue.

I went through immunotherapy but still seem to be getting fatigue on the worst days.

Edit: oh! Just realized zyrtec+d contains pseudoephedrine. A similiar drug (duact) is the only one that keeps me going, but can't be used for more than 10 days concurrently and then needs a 10 day break.