I feel bad about it, but I've decided to become an atheist?

The only reason you feel bad about it is because of the guilt and shame factor that the Christian church has been preaching for centuries. When you are old enough to make a choice, then it is free will that should rule your decision. Every son and daughter reaches a point in their life when a break from their parent's influence is necessary. It can be good, horrible or everything in between. It is not disrespect nor should you feel bad. And I wouldn't totally shut the door and call yourself an atheist. I suggest you leave the door open a crack and see what other options are out there. If you feel you've looked at all the options and don't like what you see, then go for it. All the best to you and be up front with your parents..