Hard problems are rarely just one hard problem, but until you solve the first hard sub-problem you won't even know what the other hard sub-problems are, let alone what proportion of difficulty each contributes.

It's like climbing a mountain in the fog: you've found the way forward and if it holds out you can extrapolate when you will reach the top, only to find you dead end at the face of a sheer cliff...it will be awhile before you find the next way forward.

It's progress but the destination remains further than it seems.