Why is teaching so hard?

I am sitting here in my living room while my daughter cries in her room. I realized lately that she has run of the house and she needs to remember that sometimes she needs to be contained - like in her room with all her toys (what a horrible mom I am!). But it is so hard. As she cries, so do I. I hate to see her upset, but it is for her character right? I promised myself when she was a few weeks old that my comfort would never be more important than her character. So, here I sit, praying, wishing that I could go pick her up and cuddle with her, but knowing that someday she will be able to understand.