I'm currently 13 years old, but when I was 12. I would tell my grandmother "I don't believe in God" deliberately because I wanted to make her angry, since I was angry. I didn't know it was bad to say that. I actually regret it so much that I'm about to cry writing this. But my question is: Am I going to hell for doing this, even though I asked God to forgive me? And would Matthew 10:33 refer to me?
Let's be reasonable. You did know it was a bad thing to say, that is why you said it to make your grandmother angry. However, there are two things you don't understand.
Sins will keep you out of heaven if you stay in them. Being a Christian is about changing into a better person. You're going to make a lot of mistakes in life, but if you reject the mistakes and work to become a better person, God is willing to forgive you. See: Are You Unforgivable?
The second thing is that Jesus is talking about people who say they are Christians, but will not admit it in front of other people -- particularly people who are pressuring them to deny Christ. What Jesus is saying is that we have to be committed to him no matter what happens.