Why am I such a dumb in SEO?

That's right: why am I a dumb in SEO? I am in the SEO business for 4/5 years, I know quite a lot of white hat and black hat technics (PBN, expired domain, GSA, scrape box, etc.). I read tons of books and articles but … I have had absolutely no positive results on the websites I've worked for. No massive improvements, no big jump…

It is really hopeless. Should I really give up?