Whats the hardest job when you are restoring a car

I would say staying motivated enough to NOT go for months and not doing anything to it. That can spell death to a project. But also, not to get that "deer in the headlights" look when you stare at the rolling shell that you completely stripped.

But specifically, I hate doing little trim piece jobs or trying to assemble my factory A/C without all the parts you need (push-button vacuum switches that are not available :( )