DD will be 3 in Sept. We are lucky to have friends nursing children her age, so she still does see nursing. We also have friends who no longer nurse and so she knows one day she won't nurse either. She has said she'll nurse until she's seven (she just picked a number, but it's probably true).
She likes to nurse sitting next to me, but I've explained that when we're out, I lift up my shirt and unhook my bra and that she has to be laying on my lap to nurse. I've said I get cold with my shirt up, so I ask her to help me keep it down - she's really good about that.
I generally don't use "I don't want to share them" because I hope to tandem nurse someday and will want to share them then, so figure I'll lay the ground work now!
I think that talking about nursing is best when you're both relaxed and perhaps just finished nursing, not focussing on getting to it.
By the way, I recently read Mothering Your Nursing Toddler and really liked it - could be helpful for you in feeling supported for the wonderful gift you are giving your child and help with talking with her about it.