if you fix A, BCD will break. You will fix B, only to
see E and F break,
if it really goes like that, you should keep rewriting the unit from
scratch, each time making a better design in respect to more and more and
more isolation and adding more and more and more abstraction levels. you
must use all your previous experience each time redesigning the internals.
also you must find ways to constantly fuel yourself with more patience and
motivation.
yash