So this is going to be hard to hear but sometimes it takes something like this to grow your own life. It's not being ungrateful to your parents to have your own beliefs, hopes, dreams and desires. Your definition of a good person is allowed to be different to that of your mom's. Judging from her reaction though she is shocked, I don't believe she thinks that you're a bad person. Focus on your studies, and continue to grow your own life. Continue to show your appreciation to your parents and I'm sure over time they will find comfort in what you are building for yourself.