Hmm, you have a number of different things going on. I think it would be quite hard to wean a 2 year old who co-sleeps. I would consider trying to work on moving her to her own bed before you try to wean her (better to do things gradually). Once she is in her own crib/bed, she will almost certainly sleep longer at night. I am not against co-sleeping and my son slept right next to me for the first six months of his life, but I will say that children sleep better and longer when they are just a bit separated from their parents.
Of course, she will not want to sleep on her own because she has been sleeping with you for so long. I would start by trying to get her to nap in her own room and by trying to institute a very clear bedtime routine (wash/bath, brush teeth, pjs, stories, kisses and then bed) at the same time every night. After you go through that, you could put her in her own crib and then stay with her for a certain amount of time (maybe until she falls asleep in the beginning so she isn't so upset) and then shorten the time every few nights until she feels comfortable sleeping on her own. I am a huge believer in the power of a reliable bedtime routine. My son (17 months) has had one since he was about 10 months and he goes to bed between 7 and 7:30 every night and sleeps until 7:30 or 8 in the morning.
You will need to be very patient and consistent with this process. I'm sure that she will cry. I am not generally for letting kids cry it out but she is too old now for there to be much hope that she won't cry at all. I would tolerate some tears, but stay with her so she knows that you love her even if you won't give in and take her out of her crib. I'd expect a transition like this to take a good couple of weeks.
Once she is sleeping on her own, I'd try to slowly cut down on nursings. You should feel proud that you have nursed for so long and that your daughter clearly likes to be with you and get comfort from nursing. Do you pump at all? I think the best way to introduce milk is to mix breastmilk with cow milk. You can start with 3/4 breastmilk so the taste is familiar and slowly adjust the proportions.
You will probably also need to be firm about when to nurse. You could start to insist that she is a big girl and will have to drink from a sippy cup at meal times - no more nursing during the day. I'm sure she'll protest this too (she's two after all), but be calm and consistent. No nursing during the day. Lots of hugs and cuddles and love, but no nursing. You could save nursing for right before bed as a treat for sleeping in her own bed. And you could still nurse at night if she wakes up just to save your sanity(if she is eating enough during the day though, she should start sleeping through the night very soon - 2 is very old to still be waking up).
I'd make a plan about how you want to handle all of this and take steps to change things incrementally. If you plan out what you want to do (what you think is reasonable) then you will have a better chance of being consistent with your daughter. If you are inconsistent (sometimes she can sleep with you, sometimes she can nurse during the day, etc.) she will have no idea what the real rules are. Once she understands that the rules are real, she will adjust much faster and you will have fewer tantrums and more sleep.
Good luck to you - its hard to say no to your child and very hard to deny them nursing since it is such a loving thing. But she is perfectly ready to wean and you should not feel guilty about being the mom and setting some limits.
B.