Why can I code most systems but not the safety-critical ones?

You say the complex systems defeat you. Look closer. They do not defeat your understanding — you can read the logic, trace the signal, follow the design. What defeats you is the requirement to be thorough where thoroughness is dull, to check the case that will almost certainly never happen, to write the documentation nobody will read until something breaks. This is not a harder version of what you already do. It is a different demand: patience where you want momentum, humility where you want confidence, proof where you are used to trusting your instinct. Most programmers can write correct code quickly. Few can write correct code slowly, on purpose, for years, without ever seeing the failure they prevented. So ask yourself plainly: are you missing knowledge, or are you refusing the pace the work requires? One is solved by study. The other is solved by choice, made today, and kept tomorrow.