Feeling overwhelmed is the brain's way of saying 'we've wandered too deep into the forest'. Sometimes the brain needs a break from too many 'trees' (individual specifics). It needs time to relax and look at the ocean (big picture).
I'm certainly no expert on plotting, but my common sense tells me that while there may be many guidebooks, each traveler has to plan their own route -- and that takes time. Maybe you have to try a few imperfect paths, chop down some brambles, before you find the your way.
Maybe take your conscious self for a walk in nature, and let your subconscious self do its job (plotting). The subconscious always gets there in the end if you give it time and space.
And, if none of this helps, at least you'll have had some 'oxygen to the brain' exercise which will better prepare it for when the more practical tips comes along.
All the best.