I often get most frustrated when I get to the end of a page, and have to try out new colors that are not working or colors on sound effects, shading, etc. When I have to go back and forth on trying colors or making other minor adjustments to things that aren't working, when if it was something I'd already done before and knew what to do I could do it instantly, it makes me feel like my time is being unproductively spent.
So, I've had to try to work on understanding that experimentation is also an important and necessary part of the process, but I still prefer the parts where I feel I'm working efficiently and making important progress, as opposed to what just feels like fiddling.