I want to add one other remark: I sympathize with the anxiety you're conveying about implementing this stuff. I used to feel the same way. The Bellare/Rogaway notes I linked above boosted my confidence a ton. I know perfectly well that protocol XYZ was designed by someone smarter and more experienced than me, whose engineering judgment is better than mine. So what makes me think I have any chance?

If you study those notes, you'll understand how cryptographers think, how security proofs work, and how modern protocols (though usually not older ones) are designed so that the proofs come along with them. Instead of having to rely on impossible-to-capture shades of finely tuned judgment, you get actual theorems that are bulletproof. So if you have a reasonable math background, you know what proofs are, you know that if your thing is done right then (against the designed-for threat model) your stuff is secure, and you know what it means for your thing to be done right.

So that just leaves executing the latter, which is still the usual error-prone engineering. But good languages and software (look e.g. at Cryptol on galois.com) can help with that too. Most OpenSSL CVE's lately are things like heap overflows rather than crypto misdesign. Coding in Rust is supposed to eliminate those (idk if it always does in practice).

I can't prove in a few sentences that this isn't false confidence but I think there is something to it. At the very least, understanding some theory brings a great improvement to one's clarity of thought.