What Causes Shaky And Tremble Feeling After Wearing Off Xanax?

I'm a 56 year old male, have had anxiety/panic disorder for years, I've been on 0.5 mgm of Xanax (at bedtime) for twenty years, the last two or so years I've noticed after it wears off I feel shaky jittery and tremble, could it be the tolerance build up over the years, or is it simply the anxiety is getting worse? I've tried various ssri's without much success...Thanks for any help