Your English is very good, actually!
Are you able to talk to your parents about these problems? Honestly, I would not go for a psychiatrist right away. You could probably start with a counselor who can help you determine if your problems are with your thoughts, or if you have a medical issue that needs treatment with medication.
My own experience is when i get depressed my IQ drops dramatically. It becomes difficult to think. And difficult to talk. My speach wil slow down. It will take me longer to say words and form sentences. My memory declines. I lose interest in subjects. Symptoms similar to what you describe.
The good news is when i get out of the depression and recover from it, my IQ goes back up, i become smart again, i talk normal again, and i feel much better. I become interested in things and people again.
I can't really diagnose you here, it does sound like a doctor has checked for all other reasons that could cause those symptoms, and found nothing, which is part of tge criteria for diagnosing depression, is ruling out all other possible reasons.
So it's quite possible it could be Clinical Depression, meaning it's a body thing, which can be treated with a combination of medication, (which i personaly originally thought was a bad idea, until i found the medication that worked for me, and it fixed everything, almost miraculously, so now i'm all for medication -- but need to find the one that works for you. That's the hard part. There is no one medication that works for everyone; there are many different ones which have helped others, most of them did nothing for me, i had to try them all until i found the one that worked for me), and (after that long parenthetical statement about medication) also many techniques and things one can do to help alleviate depression. Such as, get a little exercise (walk, or walk with a friend), engage in some activity you enjoy, spend time with people, friends.
Your school may have a counselor or a whole program designed to help assist students having depression. They may have groups you can join, sometimes just spending time with peers who understand and accept you can be uplifting.
I'll talk with my counselor. Sometimes just having someone (like a counselor or friend who knows how to listen) to talk to, i'll tell them all my problems, they listen, and then i'll feel better, even though we never fixed any of my problems, just teling someone about my problems helped uncouple the emotional reaction i was having about my problems. I still have all the problems, but the emotional distress over my problems is diminished, simply because i told someone about my problems and they listened.
That and medication.
I personally need both. The medication makes it possible. The self care and therapy makes it happen.
Depending on where you are, you can start by telling your regular doctor about your depression. They may prescribe some medication, or they may refer you to a psychiatrist, who is a doctor that specializes in the subject. (There's kind of a shortage of psychiatriists so regular doctors try to take up the slack.)
Parents may or may not understand, depending on their experience. Sometimes depression runs in families, so you may have a relative who understands. Likely there are adults around you who understand. It's more common than i realized.
Best wishes. And know that you won't have to spend the rest of your life suffering. There is a way out. There are people who can help.
Some studies have pointed out that frequent exposure to sunlight can help alleviate depression. On sunny morning, if you take a long walk outdoors, it will speed up your metabolism and effectively relieve the symptoms of depression. Infrared rays in the sun will dilate the capillaries and speed up blood circulation.