I have battled allergies all my life. The reason Zyrtec is no longer working for you is that you hit the limit on your current allergy meds. I was taking Clariton for a couple of years and it worked wonderful, then one year, it did nothing!!! I went to my doctor and she put me on Allegra, worked great for a while. I am now taking Zyrtec and loving it, but my guess is that after a couple of years, my body won't respond to it and I will have to move on to something else.
I have looked into allergy shots, but haven't tried them myself. Of course mine really haven't kicked in YET this season.