start with making a list.
Columns: Model, serial, year, running, what's wrong, what attachments, notes.
Once they are all on paper, order them in priority. No, they cannot all be "1"
You don't have to limit yourself to 5, but set a mental number before you do this. Parts units for the ones you are keeping don't count, but you have to look yourself in the eye and say "This is a parts unit, it will live outside and I have no intention of restoring".
then get focused on the top of the list.
I get feeling the same way you are every now and then, and that applies to everyday
life as well. As Daniel said, walk away for a bit too.
Once you stroke a couple things off the list, it's amazing how big the light at the end
of the tunnel looks.
Just make sure you keep your shop door locked, in case Kenny shows up with a truck!!