Questions about being an understudy....?

I was recently cast as the understudy for a play.Should i stick with it or quit? The reason I am thinking of quitting is because i KNOW I am better than the girl who got the part, and i worked really hard. I walked to the library to rent the dvd and cd, to the store to get ink so i could print lyrics, every day i sang all the songs and read from a script which i bought with my own money, for 2+ hours. I was so excited about it, and how do u think i felt when the girl who got the lead talked tro me saying... oh u r a dum munchkin and i am dorothy? shes all sweet to the directors but she really hurt my feelings... i thought i did really well at both the callbacks and the auditions and not to sound full of my self but i was better than her, idk