My best friend takes me for granted. wat do i do?

n she accepts it....i have discussed this with her n she says that she knows shes doin so...but she says she cant help it.....it hurts a lot wen i do stuff for her...n she doesnt even react positively....she told me that she cant be nice to me ( i dunno for wat reason!!).... n just yesterday, she said tht she doesnt deserve a friend like me n tht she is a very sucky best friend...