I would "just say no". Pick a day and be the boss. She's old enough to learn that when you say no its no.
I would, in advance of the decided day, start offering your pumped milk in her favorite sippy cup. When the day comes, offer her that at the nursing times. Hold her, if she's not so mad she rejects you, while she drinks. What she wants is the closeness nursing offers her. If you are firm about "no!", then she WILL give up eventually, probably faster than you think.
Once she is drinking breast milk from a sippy cup you can start adding in cow's milk, in very small amounts so she cant possibly notice. Add more and more as time goes on, removing your milk from the mix very gradually. My daughter needed hypoallergenic formula (she could not nurse due to health problems and pumping full time just did not work) and the stuff tasted AWFUL!! This method worked very well, getting her on the icky tasting formula within 2 weeks.
Another thing, my pediatrician said that kids do NOT need to drink milk. There are calcium fortified juices, calcuim in cheese which most kids love, yogurt, green vegetables, and many other sources. If she doesnt want to drink cow's milk its just fine. Many vegan mothers, though I am not, raise perfectly healthy kids with strong bones never giving them cow's milk.
So be the boss because its your body, cuddle her a lot- extra cuddles, and be flexible about her not liking cow's milk if she just wont drink it no matter how slowly you try to get her onto it.
Good luck!