Need help translating a few sentences.
Hello. I would like some corrections on the grammar of these sentences:

1. " il est le sorte de personne qui fut bêtement et tenir bon de son avis, peu importe de logique et si il s'est avérée erroné"

I was trying to say "He is the type of person who would foolishly stand behind his opinions, regardless of logic and if he is proven wrong".

I didn't know how say "foolishly stand behind" so instead I put "be foolish and stand behind".

2. "il a trouvé ses cours difficile et comme il manque de l'intelligence, il doit sa réussite à sa nature assidu et travailleur."

I wanted to say "he found his classes and as he lacks intelligence, he owes his assiduous and hardworking nature".

3. "de depuis s'enfance" I wanted to say "from since his childhood"

Question

Need help translating a few sentences.
Hello. I would like some corrections on the grammar of these sentences:

1. " il est le sorte de personne qui fut bêtement et tenir bon de son avis, peu importe de logique et si il s'est avérée erroné"

I was trying to say "He is the type of person who would foolishly stand behind his opinions, regardless of logic and if he is proven wrong".

I didn't know how say "foolishly stand behind" so instead I put "be foolish and stand behind".

2.  "il a trouvé ses cours difficile et comme il manque de l'intelligence, il doit sa réussite à sa nature assidu et travailleur."

I wanted to say "he found his classes and as he lacks intelligence, he owes his assiduous and hardworking nature".

3.  "de depuis s'enfance" I wanted to say "from since his childhood"

Answer

C'est la sorte de personne qui s'obstine bêtement même s'il a complètement tort
Il a trouvé ses cours difficiles, et avec son manque d'intelligence, il ne doit sa réussite qu'à son assiduité et sa nature travailleuse
Depuis son enfance