try turn off the Undo History and turn on the "skip loading" there. The undo history makes your file to become a MAMMOTH and can give problems to loading a tool. See the image below
Like others have said, split your tool, preferably make each part separate from another entirely so that you don't loose your work and make a ton of rendition backs you save throughout the process so you can at least return to a previous state. I always try to limit my undo's to 100 then delete, if they go to high Zbrush…