My dd is 22 months, was probably about 20 months when she weaned off the bedtime boob surprisingly easy - this is what worked for us:
Instead of offering the boob in the bedroom, in the dark, before sleep, I one day told her it's nearly bedtime and offered her a boob in her room. She accepted, we then had a change and read some books. Then off to bed. She asked for boob when we got to bed and I calmly said 'but you already had boob, now it's bedtime'. It worked!
The next day I offered her a boob after we had the bedtime change, just before reading books (she loves books!) she accepted but had a very short feed as she was eyeing her bookcase for the next step in the routine. No boob was asked for come bedtime.
The third day I offered boob and she said 'no, book'. That was it!
Sadly I had my hopes up that this would magically solve the other 2 boob sessions she has during the night (because boob to sleep creates association right? And that is supposedly why they wake up wanting it? Not my dd!) but it didn't so just in case you are in the same situation...
She has recently occasionally started asking for boob in the evening and smiling. I say no and smile back and that's the end of that 
Not sure if it'll work for you but might be worth a shot