I nursed my kids until they self-weaned.
My daughter self-weaned at about 2.5 years old.
Up until then, I talked to her about it.... and when she asked I would say something like "in a minute, I'm busy..." then I would make myself busy and NOT sit down... then she would get distracted. And, I would also lessen the time that she was at the boob.... not let her just LINGER there.
Then, my friends who had kids that age and nursing... actually put band-aids on their nipples. Then told their kid "mommy's milk is broken..." or "Mommy has a boo-boo...." and they said that worked for their kids.
Try get your girl a lovey as well. Something else to comfort with. Make it real SPECIAL for her...
Now your 1 boob is bigger than the other... because it has reacted to the demand upon it, by your daughter. And the other boob was not used.
Or some Moms only let their child "nurse" at certain times.... night time. Telling the child when the sun goes down, and its night time only.
Meanwhile, give her cow's milk. Is she still drinking that? Give it to her however she will take it... just to segue her off of your boobs.
You need to talk to your girl about it, in a non-threatening way. Tell her she is a big girl etc. My girl just one day stopped on her own... just like that. And then she thought it was funny that she even breastfed. And then that was the end of it all.
All the best,
Susan