Why am I so stupid and I can't think at all sometimes? It's like I'm so cold-minded, even in serious moments. My brain doesn't work fast these last two years. I haven't heard that it's wierd. I want to be back at normal like before. Please explain why, what is causing it and is it normal?
Assuming that you are not using drugs, such as marijuana or alcohol that can seriously impact your ability to think, and that you are not on a psychiatric medication, then this may be something you should discuss with your doctor.
It is true that during puberty your braining is rewiring itself and some tasks become harder for a while, but this phase generally doesn't last all that long and the majority of people don't notice.