Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh

 

Uttering words with intention behind as a mistake are not liable and Allah will not hold you responsible for that. No need to think of yourself as exited from Islam and that you need to say words of Shahadah to come back to Islam.

 

Also at age 12 when young such thoughts do not count.

 

My advice is to be busy learning about your faith and study Quran it will help you a lot not thinking much about these things.

 

Almighty Allah knows best.