I was on 10mg for a little over a year. It worked really good at first, then I started building up tolerance to it. I went from taking it and being a sleep in less than 20mins, to taking it and being a sleep in 2 hrs on a good night. I have a tendancy to build up tolerance very quickly to meds. When I was on the Ambien, I would frequently not remember what happened after taking it, but always made sure that I was not doing anything important.
Have you talked to your pdoc about your memory problems? Your dose may need to be adjusted, or you might benefit from a break off it. You really need to share with your pdoc the problems you are experiencing.