The singer said in the letter to fans: "I've actually learned to say 'NO!' with this newly found freedom.
"I'm sorry that my life seemed like it was all over the place the past two years. It's probably because it was!"
She said it was great to be married to Kevin and that she could not wait to "start my family".
Britney also wrote: "Going and going and going is all I've ever known since I was 15 years old. Times are changing and so am I."
In June she injured her knee during a music video shoot, forcing her to cancel the summer leg of her North American tour.
"I know now that my knee gave out on me this past summer so that I would have no choice but to stop," she said.
"My body was shutting down and needed rest. It's funny how the Man upstairs works."
She also wrote: "My prerogative right now is to just chill and let all the other overexposed blondes on the cover of US Weekly be your entertainment - good luck girls!"