With the rare exception, everyone 'sucks' at things like algorithms and math. That feeling of sucking and frustration is the normal feeling the vast majority of people have when trying to teach themselves or learn complex logic. Some of this can be rust i.e. you have not been thinking this way for some years. But the truth is it's also a slog.

Progress in things like algorithms and math, at least from my experience, is measured in 100s of hours, not tens of hours.

You need to 'embrace' the frustration. That's just what learning hard stuff is all about. It makes you feel stupid.

Even the successful academics all have their small list of role models, all of whom make them feel stupid (a list I think ends with Von Neuman).

fwiw I just started studying CS basics at age 29 (although I'm a data scientist professionally). I'm doing it for 'myself' though, not for interviewing.