I recommend beginning at the fundamentals. For example here's a video that walks through Diffie Hellman, so that anybody can follow, you can probably sprint through it, but by taking it slow they avoid accidentally forgetting anything important.

https://www.youtube.com/watch?v=YEBfamv-_do

Grasping the fundamentals means that when it comes to policy decisions (e.g. in the management of certificates) you can see what the consequences of a particular decision are, rather than just hoping that whoever proposed that policy knew what they were doing.

For example, I think a lot of people today use Certificate Signing Request (CSR) files without understanding them at all. But once you have a grounding in the underlying elements you can see at once what the CSR does, and why it's necessary without needing to have that spelled out separately.

Or another example, understanding what was and was not risky as a result of the known weakness of SHA-1. I saw a lot of scare-mongering by security people who saw the SHA-1 weakness as somehow meaning impossible things were now likely, but it only affected an important but quite narrow type of usage, people who understood that could make better, more careful decisions without putting anybody at risk.