You aren't alone. Historically before calculators were common, engineers and mathematicians would actually have books with basic arithmetic answers already done, or they would hire people (usually women) called 'computers' (no joke, that's what the term was used for before computers as we know it were invented) to do the basic calculations for mathematicians so they can focus on the more complicated stuff.

So even a highly talented mathematician from the 1910s and 1920s would still struggle as you do.