I think it’s more likely that Sherman was bipolar and just suffered a particularly serious episode during that period. It didn’t prevent him from winning in the end, any more than it prevented Winston Churchill from leading Britain to victory during World War II.
Or it may be that he realized what a fight was going to be necessary, and that might have made him sad. Since it was reality, to know what he knew, and not be sad would have been crazy.
Of course people who did not know what he knew would have been sure that such a fight would not be necessary, and would see his sadness as unwarranted.