I nursed four children. Two of them went to 3 years, two stopped at around 2years. During the time that they transition to "comfort nursing" more so than for nutrition can be especially challenging for mom who often sees no end in sight. But it IS a relationship, which means that you have to take into account her feelings about it as well as your own.
Think about why you want to stop. Are you uncomfortable with it? Will she not sleep without it? What are you other feelings about it? What would happen if you set a 2 year mark to stop instead of trying to stop Right Now?
Now think about what SHE gets out of the relationship--comfort, tangible love, a way to calm down and get to sleep, and it continues to be great nutrition for her! The longer a child nurses, the longer the benefits last.
HOWEVER,
I am the last person to tell you that you HAVE to nurse for X number of years. But this is a a big change for her, she has been doing this for as long as she has been a separate entity from you. I suggest you find a copy of the book The Womanly Art of Breastfeeding and read the chapters that deal with weaning. This was my bible during the 10 years that I was breastfeeding, and it helped me to wean in a gentle and non-combative way (with one exception, I was ill when I got pg with one of my children and had to abruptly wean the older child. It was HELL, and I totally regret it).
I do understand your frustration, but remember that this will be such a huge adjustment for her, the more lovingly you do it, the better all around. You CAN talk to her about it, you CAN tell her that you want to move on and stop nursing her, but please understand she is attached to this activity because it's GOOD for her. And if she is throwing tantrums it's because she doesn't know HOW to adjust to the change, she is frustrated and angry because a need is not being filled. If you want to wean her, you need to find a new way to fill that need. Good luck, mama. And be patient with her. This age is full of growth and new beginnings, tantrums are common in children who have difficulty processing change, it's your job to help her do this.