How do I tell my parents that I don't want to become a doctor anymore?

Just tell them what you wrote here, I'm quite sure they will understand how important it is to be happy with a career and life. Also, since they are you're parents, somewhere deep down (even if they act differently) they will probably always love you, sometimes it just needs a little time to get used to the 'new' you. But if you explain your situation and describe all your feelings about this, there's a big chance they will accept your decision, even if they don't really support it.