Zyrtec, for me, would be a better choice than the Singulair. Singulair can take several weeks to kick in whereas the Zyrtec should only take a day or two.
I actually take both. singulair since i have asthma that is allergy related and zyrtec for multiple allergies. the zyrtec should work immediately . since i never had any side effects to the singulair i can't comment.
I assume you are on a daily asthma med besides the Singulair...if not, then that might be what you need to add and not the Zyretc...though I took the Singulair in addition to my Zyrtec and daily inhaler for 2 months and never saw any change...
Dizziness and blocked ears can be signs of a sinus infection. And there is research showing sinus infections can exaserbate asthma. I would ask for a full sinus work-up including CT.
Also I agree that if you have allergies, you need an antihistamine like Zyrtec in addition to the Singulair.