I took Claritin 24 hours, 12 hours ago but my allergies are out of control again. Can I take half a Benadryl (diphenhydramine) to help with my itchiness?

Yes. Adding Benadryl (diphenhydramine) is fine. If your allergies are out of control you might want to consider adding medicated nasal sprays to your treatment regimen. Consider seeing an Allergist so that you can get better relief. Skin testing to determine sensitivities to indoor and outdoor allergens is useful. If you fail both medications and avoidance measures you might want to consider allergy shots.