Hello A.,
I feel your pain! My 2yo is all about the MINE! It must be the stage they are going through, terrible twos, I guess. When she does that, I calmly tell her, yes, that is yours. For example, when I put her socks and shoes on, she snatches them up and says "MINE!" I calmly tell her "yes, dear, they are your shoes and socks but mommy wants to help you put them on." If she doesn't I just tickle her feet until she gives them to me!
So point is, let her know that they are hers and she does have the right of possession for certain things, but let her know that sometimes she needs to share. If she still does it, just let it go. Its just a phase.
Good Luck!
-Leanne