Your Pediatrician is right. Not to mention at 18 mos she shouldn't need anyting before bedtime and bottles really can wreek havoc on their teeth. I took them away at the year mark, just took them away, got cool sippies and strawed cups and that was that.
She may cry but it will only last a few nights. She won't be traumatized, babies/toddlers don't like change. She will adjust it is just hard not to cave but you cannot cave.
Throw them all away, then there will be not temptation. Take her to get a fun new snuggle toy, something new and change up the routine at bedtime. It will just take a few nights and she will get a new routine down.
Don't keep the bottles at all in the house, throw them away!!!!!!!!
She does not need milk or anything before bedtime either, it is horrible on their teeth if they don't brush afterwards and it is a habit not out of need now.
Break the habit, help her with this and you have to be the strong one.
You can rock her before bed, just don't do it until she is asleep, wait until she is drowsy. Lay her down still awake but you can still rock her and comfort her. Nobody said you had to stop snuggling, rocking and relaxing before bed, but at 18 mos she does not need a bedtime milk, she does not need a bottle and she just needs you to change things around and form a new positive habit for her. Soften the lights, read a story, snuggle in, a sip of water if need be but if you offer her nothing it will be easier.
Not to mention when you start potty training in a few years the first thing that has to go is the nighttime drinks so do it now and save yourself a step then.
Good luck, hang in there, she won't be in therapy, she will adjust.