This is a hard parenting transition to make, because for the first part of their lives, we as parents gave them everything they wanted because their wants were their needs. We are now programmed to respond when they cry, and that is a good thing. But at 21 months, sure you still need to respond in some way, but not always by giving in to what she wants. No you don't need to drop everything to read "Goodnight Gorilla" for the fortieth time in two hours if you have spent good quality time with her. I think naming her feelings is excellent advice. It validates her feelings, but at the same time she can start learning that Mommy has needs (and wants!) too. Also breaking up your daily tasks into smaller chunks is great advice too. I used to just let the house go to sh*t because I would think, "Well, she's playing independantly now, but in ten minutes she'll be whining again, so I might as well not even start." But I learned that a whole sink full of dishes can be washed in ten minutes. Then we can read some books. I can vacuum the floor in another ten minutes. Then we can go outside for a while. Etc, you get the picture. It also allows you to go slowly and gently get her used to not having you jump to do what she wants when she wants.
It's all about balance, I think. Good luck!