My daughter is very sensitive too, but she got used to them. They hurt like a mofo in the beginning, so we used lots of motrin and wax. Eventually we moved to nothing. She built caluses on her gums/lips.
Every person and treatment is different, we were 6 months in a jaw spreader, and then we're going on almost 1.5 years of braces. That includes phase I and phase II. We have maybe 6-8 months left.