Boob to sleep

How do you break the habit of using the boob to sleep? Baby is 5, almost 6 months old and my milk seems to be drying up (different issue) so we have been giving her a bottle of formula at bed time because she seems to sleep better that way. Even after she is full, has been changed and ready to sleep, she still wants the boob to sleep. She's not actually eating, just likes to hold it in her mouth until she falls asleep. She wont take any pacifiers and she sees me as a giant walking boob so rocking her to sleep doesnt work. If im holding, she knows the boob is there and she will fight and cry and kick until she gets the boob. I dont mind comfort nursing once in a while but it would be really nice if she could go to sleep without being attaches to me. Whas it easy to break the habit? How did you do it? Did you replace it with a different ritual? How was the transition? How old was baby when you did this?