I just got home from the orthodontist and she told me in 3 weeks when i get them off i have to wear a retainer?

. She told me they would put a metal wire in the back of my lower teeth but I have to wear an ugly one on the top. When she told me i could have to wear it for a long time i got so poed. I was incredibly mad because 3 years with braces only to wear this stupid thing. Im also mad because I dont want to have a lisp or whatever. Is a retainer really necessary because my orthodontist told me the teeth shouldnt move after braces. I really need some information that will cool me down.