I was taking .5 mg of Xanax, I've been taking it for a year and my Doctor wanted it to be short term. She put me on the lorazepam because she says it lasts longer so I probably won't have to take so many.
My personal opinion is that Xanax is much more effective, comparably. How often were you taking the Xanax and why were you made to switch? What dose of Xanax were you taking?
One thing I can recommend is asking your doc if it's ok to take it sublingually (under the tongue). That's actually an approved route of administration that allows for a slightly quicker onset of action versus orally (especially good in the even of a panic attack or something like that).
Unless the dose of Ativan was a bit stronger, you likely will find that overall the Xanax works better. They're both benzos and both share tolerance and dependency (when taken regularly), so I guess I just don't understand the switch, especially if the Xanax was working for you.
It may just gradually kick in and reduce your stress.. I'm not sure. I can't remember what dosage mine were but they were blue and they always seemed to help me immediately. You may have built up a tolerance? In any case make sure and discuss it with your doctor or pharmacy