You've hit tolerance. Taper off very, very slowly with a longer acting benzodiazepine. See benzo.org.uk (Ashton Manual) or google the word benzodiazepine and find some benzo sites for a very slow taper plan. Cross over to klonopin or valium and taper off.
A doc put you on Xanax when you were 17!!!
abby
Xanax is short acting, and so the body builds up a tolerance to it. That's why benzos are considered addictive -- you keep needing more to do the same thing. Usually for long-term use they use klonopin, since it has less of a tolerance problem. If you go off, do it slowly and carefully and under the care of a psychiatrist who understands these meds.