Book about male lead dealing with mental health issues due to traumatic past. (Been trying to find it for 10 years now)

I have been searching for a specific book for almost 10 years now. I am starting to think I imagined it.

This is going to be an extremely vague recollection, but its worth a try. So please if anyone knows which book this is it would be absolutely amazing. I read it in Dutch. I assume it was translated, but there is also a chance it is originally Dutch. That’s also what makes this search so hard.

When I was 12 I read a book which I took from my classrooms library. The book however, discusses some very complex themes so I’m not even sure if it was meant for my age group. My description will have a lot of uncertainties considering memories can be untrustworthy, but hopefully someone can help because I have wanted to find it forever.

I never finished the book, so this information is based on the first few chapters.

What I remember is a male lead. He is dealing with some obvious mental health issues and is staying in either a mental hospital or just goes to therapy. He focuses all of his energy on not thinking about anything. Not necessarily “stop thinking about X”, but just complete derealization. A vague scene I can recall is that he goes into group therapy and before he knows it it is finished. He almost challenges himself to see how long he can not think about anything.

These current events are sometimes replaced with memories. From what I can recall is when he was a little boy he went to visit someone (father maybe?). His mother made him memorize her phone number, but he was never returned to her. I only sort of remember he was in a bad situation for a while with neglect and aggresion. Eventually he was taken away and probably in some sort of dcf situation and he started to write the phone number everywhere. Another child then realized it was a phone number and that is how they found his mother.

Something I also vaguely remember is that he was obsessed with either Mount Everest or some other form of mountains because if how quiet it is. He saw some movie about it and wanted to go there ever since he saw that movie.

So basically:

\-male lead dealing with mental health issues
\-flashbacks of traumatic childhood
\-possibly obsession with mountains