Returning to the question at hand, I think Guilliman would be consumed by utter despair. I once read a post on Reddit suggesting that the most grimdark scenario imaginable would be to brutally murder the Lion right in front of Guilliman, just to make him realize that no help is ever coming and that he will be forever alone. But what happens after that? Guilliman would simply continue to work. No matter the depths of his despair, his grief, or his rage, he would still pick up his quill and work himself to the bone just to delay the extinction of humanity for one more day. Regardless of how shattered he is, or if his spirit is completely broken, he is a man who will always fight to the bitter end and can never bring himself to give up. This is the outstanding quality of a tool meant to grind away relentlessly. A quality so good it can only be called a curse.